I have upgraded successful from ambari 2.1. to 126.96.36.199 and registered HDP 188.8.131.52 with a local repository.
I had the bug, that the successful registered version was not displayed in "Versions" Tab in ambari
After edit the ambari Database I was able to install the version 184.108.40.206 to all hosts.
Now, when I click on "Perform Upgrade", I get the window where I can choose between "Rolling Upgrade" and "Express Upgrade", but no matter what I click, the "Proceed" Button ist grayed out.
The other entries to check would be in /var/log/ambari-server/ambari-server.log on Ambari host.
Based on the above two, we can understand the issue and then decide next action