java.lang.NoClassDefFoundError: net/fabricmc/mappingio/FlatMappingVisitor
at knot//me.modmuss50.optifabric.mod.IntermediaryContextTransformer.(IntermediaryContextTransformer.java:86)
at knot//me.modmuss50.optifabric.mod.OptifineSetup$2.lambda$load$9(OptifineSetup.java:324)
at knot//me.modmuss50.optifabric.mod.ContextualMappingProvider.setContextTransformer(ContextualMappingProvider.java:72)
at knot//me.modmuss50.optifabric.mod.OptifineSetup$2.load(OptifineSetup.java:324)
at knot//me.modmuss50.optifabric.shadow.tinyremapper.TinyRemapper.apply$3041763d$58bfb62c(TinyRemapper.java:7703)
at knot//me.modmuss50.optifabric.mod.OptifineSetup.remapOptifine(OptifineSetup.java:3903)
at knot//me.modmuss50.optifabric.mod.OptifineSetup.remapOptifine(OptifineSetup.java:237)
at knot//me.modmuss50.optifabric.mod.OptifineSetup.getRuntime(OptifineSetup.java:185)
at knot//me.modmuss50.optifabric.mod.OptifabricSetup.run(OptifabricSetup.java:46)
at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)
at knot//com.chocohead.mm.Plugin.getMixins(Plugin.java:340)
at org.spongepowered.asm.mixin.transformer.PluginHandle.getMixins(PluginHandle.java:128)
at org.spongepowered.asm.mixin.transformer.MixinConfig.postInitialise(MixinConfig.java:877)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.prepareConfigs(MixinProcessor.java:568)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.select(MixinProcessor.java:462)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.checkSelect(MixinProcessor.java:438)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:290)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClass(MixinTransformer.java:234)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClassBytes(MixinTransformer.java:202)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.getPostMixinClassByteArray(KnotClassDelegate.java:422)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.tryLoadClass(KnotClassDelegate.java:323)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.loadClass(KnotClassDelegate.java:218)
at net.fabricmc.loader.impl.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:119)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:526)
at java.base/java.lang.Class.forName0(Native Method)
at java.base/java.lang.Class.forName(Class.java:534)
at java.base/java.lang.Class.forName(Class.java:513)
at net.fabricmc.loader.impl.util.DefaultLanguageAdapter.create(DefaultLanguageAdapter.java:50)
at net.fabricmc.loader.impl.entrypoint.EntrypointStorage$NewEntry.getOrCreate(EntrypointStorage.java:124)
at net.fabricmc.loader.impl.entrypoint.EntrypointContainerImpl.getEntrypoint(EntrypointContainerImpl.java:53)
at net.fabricmc.loader.impl.FabricLoaderImpl.invokeEntrypoints(FabricLoaderImpl.java:399)
at net.fabricmc.loader.impl.launch.knot.Knot.init(Knot.java:160)
at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:68)
at net.fabricmc.loader.impl.launch.knot.KnotClient.main(KnotClient.java:23)
Caused by: java.lang.ClassNotFoundException: net.fabricmc.mappingio.FlatMappingVisitor
at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:641)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:526)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.loadClass(KnotClassDelegate.java:226)
at net.fabricmc.loader.impl.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:119)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:526)
... 34 more

Which MC Launcher?
Which Fabric Loader version?
List of Mods AND their versions?

Which MC Launcher? Which Fabric Loader version? List of Mods AND their versions?

legacy launcher
fabric 16.9
list of mods:
DistantHorizons-2.2.1-a
fabric-api-0.110.0
firstperson-2.4.5
jei-19.21.0.247
Optifabric-Mod-Fabric-1.21.1
OptiFine_1.21.1_HD_U_J1
shulkerboxtooltip-5.1.2

Two things:

1. Support is not provided for illegal launchers and would suggest you use a legal launcher instead.
2. There is currently no Optifacbric mod available for 1.21+

If this has solved your issue for now, please close this ticket.

How to download it??

How to download it??

If you need support please open your own ticket and stop hijacking other's tickets.