Why won't Latest version of Fabric APi work in 25w31a?
It says 25w31a is WRONG version.

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Why won't Latest version of Fabric APi work in 25w31a? It says 25w31a is WRONG version.

To answer your question:
Currently, 25w31a is a snapshot. The latest version of Fabric API was specifically made for 1.21.8.
Edit: FFAPI is now available for 25w31a.