Hi, I was having this problem about 1 week ago and I can't find the solution.

Below I leave the complete error code:

---- Minecraft Crash Report ----
// I bet Cylons wouldn't have this problem.

Time: 20/12/22 05:15
Description: Rendering overlay

java.lang.NoSuchFieldError: children
    at com.bobmowzie.mowziesmobs.client.model.tools.MowzieGeoBuilder.constructBone(MowzieGeoBuilder.java:48) ~[?:1.5.25] {re:classloading}
    at software.bernie.geckolib3.geo.render.GeoBuilder.constructGeoModel(GeoBuilder.java:37) ~[?:3.0.106] {re:classloading}
    at software.bernie.geckolib3.file.GeoModelLoader.loadModel(GeoModelLoader.java:30) ~[?:3.0.106] {re:classloading}
    at software.bernie.geckolib3.resource.GeckoLibCache.lambda$reload$1(GeckoLibCache.java:69) ~[?:3.0.106] {re:classloading}
    at software.bernie.geckolib3.resource.GeckoLibCache$$Lambda$5828/726235544.apply(Unknown Source) ~[?:?] {}
    at software.bernie.geckolib3.resource.GeckoLibCache.lambda$null$5(GeckoLibCache.java:86) ~[?:3.0.106] {re:classloading}
    at software.bernie.geckolib3.resource.GeckoLibCache$$Lambda$5888/1620570388.get(Unknown Source) ~[?:?] {}
    at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1582) ~[?:1.8.0_51] {}
    at java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1574) ~[?:1.8.0_51] {}
    at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289) ~[?:1.8.0_51] {}
    at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056) ~[?:1.8.0_51] {}
    at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1689) ~[?:1.8.0_51] {re:computing_frames}
    at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157) ~[?:1.8.0_51] {}

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

-- Head --
Thread: Render thread
Stacktrace:
    at net.minecraft.client.renderer.GameRenderer.func_195458_a(GameRenderer.java:484) ~[?:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
-- Overlay render details --
Details:
    Overlay name: net.minecraft.client.gui.ResourceLoadProgressGui
Stacktrace:
    at net.minecraft.client.renderer.GameRenderer.func_195458_a(GameRenderer.java:484) ~[?:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
    at net.minecraft.client.Minecraft.func_195542_b(Minecraft.java:977) [?:?] {re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:A,pl:runtimedistcleaner:A}
    at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:607) [?:?] {re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:A,pl:runtimedistcleaner:A}
    at net.minecraft.client.main.Main.main(Main.java:184) [?:?] {re:classloading,pl:runtimedistcleaner:A}
    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:51) [forge-1.16.5-36.2.34.jar:36.2] {}
    at net.minecraftforge.fml.loading.FMLClientLaunchProvider$$Lambda$474/235195640.call(Unknown Source) [forge-1.16.5-36.2.34.jar:36.2] {}
    at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) [modlauncher-8.1.3.jar:?] {}
    at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54) [modlauncher-8.1.3.jar:?] {}
    at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72) [modlauncher-8.1.3.jar:?] {}
    at cpw.mods.modlauncher.Launcher.run(Launcher.java:82) [modlauncher-8.1.3.jar:?] {}
    at cpw.mods.modlauncher.Launcher.main(Launcher.java:66) [modlauncher-8.1.3.jar:?] {}

-- System Details --
Details:
    Minecraft Version: 1.16.5
    Minecraft Version ID: 1.16.5
    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: 1634664352 bytes (1558 MB) / 2147483648 bytes (2048 MB) up to 2147483648 bytes (2048 MB)
    CPUs: 8
    JVM Flags: 10 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xss1M -XX:+IgnoreUnrecognizedVMOptions -Xmx2G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M
    ModLauncher: 8.1.3+8.1.3+main-8.1.x.c94d18ec
    ModLauncher launch target: fmlclient
    ModLauncher naming: srg
    ModLauncher services: 
        /mixin-0.8.4.jar mixin PLUGINSERVICE 
        /eventbus-4.0.0.jar eventbus PLUGINSERVICE 
        /forge-1.16.5-36.2.34.jar object_holder_definalize PLUGINSERVICE 
        /forge-1.16.5-36.2.34.jar runtime_enum_extender PLUGINSERVICE 
        /accesstransformers-3.0.1.jar accesstransformer PLUGINSERVICE 
        /forge-1.16.5-36.2.34.jar capability_inject_definalize PLUGINSERVICE 
        /forge-1.16.5-36.2.34.jar runtimedistcleaner PLUGINSERVICE 
        /mixin-0.8.4.jar mixin TRANSFORMATIONSERVICE 
        /forge-1.16.5-36.2.34.jar fml TRANSFORMATIONSERVICE 
    FML: 36.2
    Forge: net.minecraftforge:36.2.34
    FML Language Providers: 
        [email protected]
        minecraft@1
    Mod List: 
        forge-1.16.5-36.2.34-client.jar                   |Minecraft                     |minecraft                     |1.16.5              |SIDED_SETU|Manifest: NOSIGNATURE
        useful_backpacks-1.16.5-1.12.1.90.jar             |Useful Backpacks              |usefulbackpacks               |1.12.1.90           |SIDED_SETU|Manifest: f4:a6:0b:ee:cb:8a:1a:ea:9f:9d:45:91:8f:8b:b3:ae:26:f3:bf:05:86:1d:90:9e:f6:32:2a:1a:ed:1d:ce:b0
        mowziesmobs-1.5.25.jar                            |Mowzie's Mobs                 |mowziesmobs                   |1.5.25              |COMMON_SET|Manifest: NOSIGNATURE
        blue_skies-1.16.5-1.1.3.jar                       |Blue Skies                    |blue_skies                    |1.1.3               |COMMON_SET|Manifest: NOSIGNATURE
        u_team_core-1.16.5-3.2.1.229.jar                  |U Team Core                   |uteamcore                     |3.2.1.229           |SIDED_SETU|Manifest: f4:a6:0b:ee:cb:8a:1a:ea:9f:9d:45:91:8f:8b:b3:ae:26:f3:bf:05:86:1d:90:9e:f6:32:2a:1a:ed:1d:ce:b0
        infernal-expansion-1.16.5-2.5.0.jar               |Infernal Expansion            |infernalexp                   |2.5.0               |SIDED_SETU|Manifest: NOSIGNATURE
        PickUpNotifier-v1.2-1.16.3.jar                    |Pick Up Notifier              |pickupnotifier                |1.2                 |SIDED_SETU|Manifest: d3:cc:6f:1b:30:87:fa:d9:8f:91:15:20:27:63:95:aa:d5:cb:1b:5b:e6:36:cc:57:20:a9:b3:d4:d5:1a:5d:b8
        goblintraders-1.7.3-1.16.5.jar                    |Goblin Traders                |goblintraders                 |1.7.3               |SIDED_SETU|Manifest: NOSIGNATURE
        forge-1.16.5-36.2.34-universal.jar                |Forge                         |forge                         |36.2.34             |SIDED_SETU|Manifest: 22:af:21:d8:19:82:7f:93:94:fe:2b:ac:b7:e4:41:57:68:39:87:b1:a7:5c:c6:44:f9:25:74:21:14:f5:0d:90
        Cataclysm-0.20.jar                                |Cataclysm Mod                 |cataclysm                     |1.0                 |SIDED_SETU|Manifest: NOSIGNATURE
        geckolib-forge-1.16.5-3.0.106.jar                 |GeckoLib                      |geckolib3                     |3.0.106             |SIDED_SETU|Manifest: NOSIGNATURE
        betterendforge-1.16.5-2.5.jar                     |BetterEnd Forge               |betterendforge                |1.16.5-2.5          |SIDED_SETU|Manifest: NOSIGNATURE
        structure_gel-1.16.5-1.7.8.jar                    |Structure Gel API             |structure_gel                 |1.7.8               |SIDED_SETU|Manifest: NOSIGNATURE
        DungeonsArise-1.16.5-2.1.49-beta.jar              |When Dungeons Arise           |dungeons_arise                |2.1.49              |SIDED_SETU|Manifest: NOSIGNATURE
        citadel-1.8.1-1.16.5.jar                          |Citadel                       |citadel                       |1.8.1               |COMMON_SET|Manifest: NOSIGNATURE
    Crash Report UUID: e465c5cb-030b-414e-82ba-cd5e129b4c67
    Launched Version: 1.16.5-forge-36.2.34
    Backend library: LWJGL version 3.2.2 build 10
    Backend API: AMD Radeon(TM) RX Vega 11 Graphics GL version 4.6.0 Compatibility Profile Context 22.20.27.07.221020, ATI Technologies Inc.
    GL Caps: Using framebuffer using OpenGL 3.0
    Using VBOs: Yes
    Is Modded: Definitely; Client brand changed to 'forge'
    Type: Client (map_client.txt)
    Graphics mode: fast
    Resource Packs: vanilla
    Current Language: English (US)
    CPU: 8x AMD Ryzen 5 3400G with Radeon Vega Graphics 

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