Error Caused by: java.lang.NoClassDefFoundError

I made a modpack today and my client keeps crashing and idk why.
Here's the log:

[19:25:27] [main/ERROR] [LaunchWrapper/]: Unable to launch
java.lang.reflect.InvocationTargetException: null
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_251]
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_251]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_251]
    at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_251]
    at net.minecraft.launchwrapper.Launch.launch(Launch.java:135) [launchwrapper-1.12.jar:?]
    at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [launchwrapper-1.12.jar:?]
Caused by: java.lang.NoClassDefFoundError: net/minecraft/client/gui/GuiScreenWorking
    at net.minecraft.client.main.Main.main(SourceFile:38) ~[Main.class:?]
    ... 6 more
Caused by: java.lang.ClassNotFoundException: net.minecraft.client.gui.GuiScreenWorking
    at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:191) ~[launchwrapper-1.12.jar:?]
    at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_251]
    at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_251]
    at net.minecraft.client.main.Main.main(SourceFile:38) ~[Main.class:?]
    ... 6 more
Caused by: java.lang.NoClassDefFoundError: net/minecraft/util/IProgressUpdate
    at java.lang.ClassLoader.defineClass1(Native Method) ~[?:1.8.0_251]
    at java.lang.ClassLoader.defineClass(Unknown Source) ~[?:1.8.0_251]
    at java.security.SecureClassLoader.defineClass(Unknown Source) ~[?:1.8.0_251]
    at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:182) ~[launchwrapper-1.12.jar:?]
    at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_251]
    at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_251]
    at net.minecraft.client.main.Main.main(SourceFile:38) ~[Main.class:?]
    ... 6 more
Caused by: java.lang.ClassNotFoundException: net.minecraft.util.IProgressUpdate
    at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:191) ~[launchwrapper-1.12.jar:?]
    at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_251]
    at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_251]
    at java.lang.ClassLoader.defineClass1(Native Method) ~[?:1.8.0_251]
    at java.lang.ClassLoader.defineClass(Unknown Source) ~[?:1.8.0_251]
    at java.security.SecureClassLoader.defineClass(Unknown Source) ~[?:1.8.0_251]
    at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:182) ~[launchwrapper-1.12.jar:?]
    at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_251]
    at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_251]
    at net.minecraft.client.main.Main.main(SourceFile:38) ~[Main.class:?]
    ... 6 more
Caused by: java.lang.ClassFormatError: Illegal field modifiers in class net/minecraft/util/IProgressUpdate: 0x1A
    at java.lang.ClassLoader.defineClass1(Native Method) ~[?:1.8.0_251]
    at java.lang.ClassLoader.defineClass(Unknown Source) ~[?:1.8.0_251]
    at java.security.SecureClassLoader.defineClass(Unknown Source) ~[?:1.8.0_251]
    at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:182) ~[launchwrapper-1.12.jar:?]
    at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_251]
    at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_251]
    at java.lang.ClassLoader.defineClass1(Native Method) ~[?:1.8.0_251]
    at java.lang.ClassLoader.defineClass(Unknown Source) ~[?:1.8.0_251]
    at java.security.SecureClassLoader.defineClass(Unknown Source) ~[?:1.8.0_251]
    at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:182) ~[launchwrapper-1.12.jar:?]
    at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_251]
    at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_251]
    at net.minecraft.client.main.Main.main(SourceFile:38) ~[Main.class:?]
    ... 6 more
[19:25:27] [main/INFO] [STDERR/]: [java.lang.ThreadGroup:uncaughtException:-1]: net.minecraftforge.fml.relauncher.FMLSecurityManager$ExitTrappedException
[19:25:27] [main/INFO] [STDERR/]: [java.lang.ThreadGroup:uncaughtException:-1]:     at net.minecraftforge.fml.relauncher.FMLSecurityManager.checkPermission(FMLSecurityManager.java:49)
[19:25:27] [main/INFO] [STDERR/]: [java.lang.ThreadGroup:uncaughtException:-1]:     at java.lang.SecurityManager.checkExit(Unknown Source)
[19:25:27] [main/INFO] [STDERR/]: [java.lang.ThreadGroup:uncaughtException:-1]:     at java.lang.Runtime.exit(Unknown Source)
[19:25:27] [main/INFO] [STDERR/]: [java.lang.ThreadGroup:uncaughtException:-1]:     at java.lang.System.exit(Unknown Source)
[19:25:27] [main/INFO] [STDERR/]: [java.lang.ThreadGroup:uncaughtException:-1]:     at net.minecraft.launchwrapper.Launch.launch(Launch.java:138)
[19:25:27] [main/INFO] [STDERR/]: [java.lang.ThreadGroup:uncaughtException:-1]:     at net.minecraft.launchwrapper.Launch.main(Launch.java:28)
Pls help