Usenapp requires access to the Application Support path because that is the location where it stored all database files, etc.
If the Application Support path is not accessible, Usenapp cannot be run.
The Application Support path located at:
/Users/[username]/Library/Application Support/Usenapp
If your system lost this path, you will receive that error message in Usenapp.
In that case, check the path manually to find out what's going on.