Sorry for the late reply, I had somehow missed your request.
Very likely you installed the BlenderFDS addon on Blender 3.1. This version does not work and it was skipped.
The latest BlenderFDS 6.0.0-beta1 work only with Blender 3.2 beta, as stated in the documentation and the release notes.
Let me know if it works with Blender 3.2 beta.
Emanuele