Fabric is a loader, not a compatibility mode
A mod compiled for another loader cannot be made Fabric-compatible just by moving the file into a Fabric mods folder. The project must provide a compatible build or port.
Match three things at the same time
Verify Minecraft version, loader family and dependencies. A correct loader with the wrong game version can still fail immediately.
Avoid mixed-loader troubleshooting loops
If the instance contains mods from different loader ecosystems, create a clean test profile and add only files built for the selected loader.