so i downloaded forge 1.15.2 but a lot of the mods i want to use aren't compatible with 1.15.2 yet so i downloaded forge 1.14.4 and some mods compatible with 1.14.4 . i went to my minecraft launcher and switched it to forge 1.14.4 and tried loading it but minecraft wont open and it crashes. i copy pasted the crash report so i hope that helps.

Spoiler

---- Minecraft Crash Report ----
// Don't do that.

Time: 3/30/20 10:30 PM
Description: Rendering overlay

java.lang.NullPointerException: Rendering overlay
    at net.minecraft.client.renderer.FluidBlockRenderer.func_178268_a(FluidBlockRenderer.java:41) ~[?:?] {re:classloading,xf:OptiFine:default}
    at net.minecraft.client.renderer.BlockRendererDispatcher.func_195410_a(BlockRendererDispatcher.java:154) ~[?:?] {re:classloading,xf:OptiFine:default}
    at net.minecraft.resources.IResourceManagerReloadListener.lambda$reload$0(IResourceManagerReloadListener.java:16) ~[?:?] {re:classloading}
    at net.minecraft.resources.IResourceManagerReloadListener$$Lambda$2675/1618425865.run(Unknown Source) ~[?:?] {}
    at java.util.concurrent.CompletableFuture.uniRun(CompletableFuture.java:705) ~[?:1.8.0_51] {}
    at java.util.concurrent.CompletableFuture$UniRun.tryFire(CompletableFuture.java:687) ~[?:1.8.0_51] {}
    at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:442) ~[?:1.8.0_51] {}
    at net.minecraft.resources.AsyncReloader.func_219557_a(SourceFile:71) ~[?:?] {re:classloading}
    at net.minecraft.resources.AsyncReloader$$Lambda$2768/371090257.run(Unknown Source) ~[?:?] {}
    at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213166_h(ThreadTaskExecutor.java:170) ~[?:?] {re:classloading,pl:accesstransformer:B,xf:OptiFine:default}
    at net.minecraft.util.concurrent.RecursiveEventLoop.func_213166_h(SourceFile:23) ~[?:?] {re:classloading}
    at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213168_p(ThreadTaskExecutor.java:135) ~[?:?] {re:classloading,pl:accesstransformer:B,xf:OptiFine:default}
    at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213160_bf(ThreadTaskExecutor.java:115) ~[?:?] {re:classloading,pl:accesstransformer:B,xf:OptiFine:default}
    at net.minecraft.client.Minecraft.func_195542_b(Minecraft.java:861) [?:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
    at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:384) [?:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
    at net.minecraft.client.main.Main.main(SourceFile:155) [?:?] {re:classloading}
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_51] {}
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_51] {}
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_51] {}
    at java.lang.reflect.Method.invoke(Method.java:497) ~[?:1.8.0_51] {}
    at net.minecraftforge.fml.loading.FMLClientLaunchProvider.lambda$launchService$0(FMLClientLaunchProvider.java:56) [forge-1.14.4-28.2.3.jar:28.2] {}
    at net.minecraftforge.fml.loading.FMLClientLaunchProvider$$Lambda$407/1895054149.call(Unknown Source) [forge-1.14.4-28.2.3.jar:28.2] {}
    at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) [modlauncher-4.1.0.jar:?] {}
    at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54) [modlauncher-4.1.0.jar:?] {}
    at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72) [modlauncher-4.1.0.jar:?] {}
    at cpw.mods.modlauncher.Launcher.run(Launcher.java:81) [modlauncher-4.1.0.jar:?] {}
    at cpw.mods.modlauncher.Launcher.main(Launcher.java:65) [modlauncher-4.1.0.jar:?] {}

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Thread: Client thread
Stacktrace:
    at net.minecraft.client.renderer.FluidBlockRenderer.func_178268_a(FluidBlockRenderer.java:41)
    at net.minecraft.client.renderer.BlockRendererDispatcher.func_195410_a(BlockRendererDispatcher.java:154)
    at net.minecraft.resources.IResourceManagerReloadListener.lambda$reload$0(IResourceManagerReloadListener.java:16)
    at net.minecraft.resources.IResourceManagerReloadListener$$Lambda$2675/1618425865.run(Unknown Source)
    at java.util.concurrent.CompletableFuture.uniRun(CompletableFuture.java:705)
    at java.util.concurrent.CompletableFuture$UniRun.tryFire(CompletableFuture.java:687)
    at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:442)
    at net.minecraft.resources.AsyncReloader.func_219557_a(SourceFile:71)
    at net.minecraft.resources.AsyncReloader$$Lambda$2768/371090257.run(Unknown Source)
    at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213166_h(ThreadTaskExecutor.java:170)
    at net.minecraft.util.concurrent.RecursiveEventLoop.func_213166_h(SourceFile:23)
    at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213168_p(ThreadTaskExecutor.java:135)

-- Overlay render details --
Details:
    Overlay name: net.minecraft.client.gui.ResourceLoadProgressGui
Stacktrace:
    at net.minecraft.client.renderer.GameRenderer.func_195458_a(GameRenderer.java:880)
    at net.minecraft.client.Minecraft.func_195542_b(Minecraft.java:890)
    at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:384)
    at net.minecraft.client.main.Main.main(SourceFile:155)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at net.minecraftforge.fml.loading.FMLClientLaunchProvider.lambda$launchService$0(FMLClientLaunchProvider.java:56)
    at net.minecraftforge.fml.loading.FMLClientLaunchProvider$$Lambda$407/1895054149.call(Unknown Source)
    at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37)
    at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54)
    at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72)
    at cpw.mods.modlauncher.Launcher.run(Launcher.java:81)
    at cpw.mods.modlauncher.Launcher.main(Launcher.java:65)

-- System Details --
Details:
    Minecraft Version: 1.14.4
    Minecraft Version ID: 1.14.4
    Operating System: Windows 10 (amd64) version 10.0
    Java Version: 1.8.0_51, Oracle Corporation
    Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
    Memory: 1366097024 bytes (1302 MB) / 2147483648 bytes (2048 MB) up to 2147483648 bytes (2048 MB)
    CPUs: 4
    JVM Flags: 9 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xss1M -Xmx2G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M
    ModLauncher: 4.1.0+62+5bfa59b
    ModLauncher launch target: fmlclient
    ModLauncher naming: srg
    ModLauncher services: 
        /eventbus-1.0.0-service.jar eventbus PLUGINSERVICE 
        /forge-1.14.4-28.2.3.jar object_holder_definalize PLUGINSERVICE 
        /forge-1.14.4-28.2.3.jar runtime_enum_extender PLUGINSERVICE 
        /accesstransformers-1.0.5-shadowed.jar accesstransformer PLUGINSERVICE 
        /forge-1.14.4-28.2.3.jar capability_inject_definalize PLUGINSERVICE 
        /forge-1.14.4-28.2.3.jar runtimedistcleaner PLUGINSERVICE 
        /OptiFine_1.14.4_HD_U_F5.jar OptiFine TRANSFORMATIONSERVICE 
        /forge-1.14.4-28.2.3.jar fml TRANSFORMATIONSERVICE 
    FML: 28.2
    Forge: net.minecraftforge:28.2.3
    FML Language Providers: 
        [email protected]
        minecraft@1
    Mod List: 
        forge-1.14.4-28.2.3-client.jar Minecraft {[email protected] DONE}
        fairylights-2.1.15-1.14.4.jar Fairy Lights {[email protected] DONE}
        forge-1.14.4-28.2.3-universal.jar Forge {[email protected] DONE}
        BiomesOPlenty-1.14.4-9.0.0.262-universal.jar Biomes O' Plenty {[email protected] DONE}
        Xaeros_Minimap_20.5.2_Forge_1.14.4.jar Xaero's Minimap {[email protected] DONE}
        jei-1.14.4-6.0.0.27.jar Just Enough Items {[email protected] DONE}
    Launched Version: 1.14.4-forge-28.2.3
    LWJGL: 3.2.2 build 10
    OpenGL: AMD Radeon(TM) R2 Graphics GL version 4.5.13418 Compatibility Profile Context 15.301.2301.1002, ATI Technologies Inc.
    GL Caps: Using GL 1.3 multitexturing.
Using GL 1.3 texture combiners.
Using framebuffer objects because OpenGL 3.0 is supported and separate blending is supported.
Shaders are available because OpenGL 2.1 is supported.
VBOs are available because OpenGL 1.5 is supported.

    Using VBOs: Yes
    Is Modded: Definitely; Client brand changed to 'forge'
    Type: Client (map_client.txt)
    Resource Packs: programer_art, vanilla
    Current Language: English (US)
    CPU: 4x AMD E2-7110 APU with AMD Radeon R2 Graphics 
    OptiFine Version: OptiFine_1.14.4_HD_U_F5
    OptiFine Build: 20191204-141250
    Render Distance Chunks: 16
    Mipmaps: 4
    Anisotropic Filtering: 1
    Antialiasing: 0
    Multitexture: false
    Shaders: null
    OpenGlVersion: 4.5.13418 Compatibility Profile Context 15.301.2301.1002
    OpenGlRenderer: AMD Radeon(TM) R2 Graphics
    OpenGlVendor: ATI Technologies Inc.
    CpuCount: 4

 

Optifine needs specific Forge version.

In this case you need to use Forge 28.1.56.