When I ran into this issue, my first thought was to search Stack Overflow for solution. I did the search, found several topics. But, unlike my issue, those posters got
iTunes connect (or at least some services that iTunes Connect uses) are down for the moment, so "Take a break for an hour, try again ..." is your best shot. Apple of course doesn't update their status page until tomorrow or so.
https://developer.apple.com/system-status/