I am having a similar problem with Office 365 (I upgraded to Office 2016) and a Project Professional 2013 that I purchased separately. Upgrading to Office 2016 would not work unless I uninstalled Project Profession 2013. I found the Special Offer For customers
and have talked with 4 people at Microsoft but no one would help with the problem.
Thank you for any help you can provide.
Not sure how to advise you. I had the very same issue the first time.
I mentioned that when I took the link from the Offer Site and went to the Billing Chat. One of the first things I did was paste the Offer URL in the Chat Window. This time it went real fast.
You could try that, and ask them to respond to the Offer at the URL, that you qualify for.
Good Luck.