The type of t doesn’t quite look right to me there.
If you want to use Fin, the best way is to only Require it. I.e. From Coq.Vectors Require Fin. Then you can write Fin.t. If you still want to use fin you can declare an abbreviation:
This would definitely be a great feature to have. Another workaround is to assign a module to a local module so you can use then in a shorter notation. but this is definitely not ideal.