What it means
Fabric checks every mod's requirements before starting. If something doesn't fit, it stops with a list:
Incompatible mods found!
A potential solution has been determined:
- Install fabric-api, any version.
- Replace mod 'Sodium' (sodium) 0.5.8 with any version that is compatible with minecraft 1.21.4
Unlike most crashes, this one usually tells you exactly what to do. Read the list.
The common ones
"Install fabric-api, any version": almost every Fabric mod needs Fabric API. Install it from the Mods tab, matching your Minecraft version.
"Replace mod ... with any version that is compatible with minecraft X": that mod is for a different Minecraft version. Install the version for yours, or remove it.
"Install [some library]": a mod depends on a library such as Cloth Config, Architectury or Fabric Language Kotlin. Install it from the Mods tab.
"Remove mod X" or "X is incompatible with Y": two mods can't run together. Keep the one you need more.
"requires version X of fabricloader": the mod needs a newer Fabric Loader. Update the loader on the Startup tab.
The fix
- Read each line of "A potential solution has been determined".
- Do what it says, one line at a time, using the Mods tab or the
modsfolder in the Files tab. - Restart. Repeat if a new list appears.
Quilt and NeoForge mods
A Forge or NeoForge mod in a Fabric server's mods folder can't be loaded at all and can produce confusing messages. Check every file is a Fabric build.
Questions
I installed Fabric API but it still asks for it.
Check you installed the build for your server's Minecraft version, and that it's in the mods folder, not a subfolder.
Does every player need the same mods?
Mods that add content (blocks, items, mobs) must be on both the server and every player's game. Server-only mods such as Lithium don't need to be on players' games.