Hello,
It seems like there is an issue with your SharePoint Site. Please try to create a blank site and see you can add it there.
There is one bug that you might be running into though. If you had SharePoint 2010, and upgraded to SharePoint 2013, there is a good chance that your SharePoint Site is still running with the SharePoint 2010 template. There is a bug preventing us from installing on Sites with the SharePoint 2010 Experience:
BUG-000086920 : Installer fails for SP2013 on sites created with 2010 experience