Hello, unfortunately we really don't know what the underlying issue could be. Perhaps it's only an error on the "test" button, and the plug-in will work?
This is not an Inedo-created plugin, and we're not familiar with Java or Jenkins to know how to to debug it. It should still work (and I know we have some users who got it working), but otherwise we don't really know how to debug it I'm afraid :(
The source code is here, but I don't know if it's at all helpful:
https://github.com/jenkinsci/inedo-proget-plugin
Cheers,
Alana