Hi. I am having the same issue trying to install another Pro license on a different site. You were able to somehow install it successfully for the other site, now I’m having the same issues here. Here is a repeat of my last message. 
We just installed a fresh copy of Pro at our client’s site and when we go to validate it, after entering the correct key, it says " Congratulations, your site is now validated!" but then the screen comes back to the message that says " You’re almost finished! Your license of Pro is not validated . "
Oddly, when I check my registered sites, this site DOES show up there! I’ve tried deleting the license and re-validating but it does the same thing. I’ve also tried deactivating all plugins, but no change.
Thank you for any assistance!