This solution worked for me. Go to https://developer.apple.com/account/resources/identifiers/list Refresh the page. Then click add and follow the wizard. Bug happens if you followed add link directly without going to the list first, some js libraries not being loaded this way.

I noticed that the update takes a long time, and when the screen sleeps, the wifi connection disconnects and the installation gets stuck. 1 - I disabled the screen sleep from battery options https://mackeeper.com/blog/prevent-your-mac-from-sleep/ 2 - I cleared the cache from ~/Library/Caches, 3 - Restarted the machine and tried again. I left the update overnight, internet speed is 100Mbps but still takes hours for update/install. In the morning I found the latest version installed. Hope this helps.