using v0.6.21a with Fabric API 0.124.4 on MC version 25w21a.
here's the main part of the crash report:

java.lang.IllegalStateException: Can only blur once per frame at knot//net.minecraft.class_11246.method_71299(class_11246.java:46) at knot//net.minecraft.class_332.method_71278(class_332.java:111) at knot//net.minecraft.class_437.method_57734(class_437.java:366) at knot//net.minecraft.class_437.method_25420(class_437.java:359) at knot//com.tom.cpm.client.GuiImpl.method_25394(GuiImpl.java:93) at knot//net.minecraft.class_437.method_47413(class_437.java:113) at knot//net.minecraft.class_757.mixinextras$bridge$method_47413$53(class_757.java) at knot//net.minecraft.class_757.wrapOperation$znj000$fabric-screen-api-v1$onRenderScreen(class_757.java:923) at knot//net.minecraft.class_757.method_3192(class_757.java:549) at knot//net.minecraft.class_310.method_1523(class_310.java:1355) at knot//net.minecraft.class_310.method_1514(class_310.java:941) at knot//net.minecraft.client.main.Main.main(Main.java:265) at net.fabricmc.loader.impl.game.minecraft.MinecraftGameProvider.launch(MinecraftGameProvider.java:480) at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:74) at net.fabricmc.loader.impl.launch.knot.KnotClient.main(KnotClient.java:23) at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103) at java.base/java.lang.reflect.Method.invoke(Method.java:580) at org.multimc.onesix.OneSixLauncher.launchWithMainClass(OneSixLauncher.java:243) at org.multimc.onesix.OneSixLauncher.launch(OneSixLauncher.java:278) at org.multimc.EntryPoint.listen(EntryPoint.java:143) at org.multimc.EntryPoint.main(EntryPoint.java:34)

You are using the wrong version, please use this:
https://modrinth.com/plugin/custom-player-models/version/1.21.6v0.6.22a-fabric