Hi there,
There’s a couple reasons why the Site Library wouldn’t show:
1. You’re using an old version (< 5.4) of PHP.
2. Your server isn’t able to communicate with the server where the sites are served from.
Since your server is allowing you to connect with generatepress.com to activate your license key, I would assume #2 is the issue.
I would contact your hosting and make sure your server is able to talk to generatepress.com and gpsites.co.