Update
Well, something quite strange happened. When the transaction got confirmed, which took about 30 minutes, I received the key in my email and tried using it, but it didn't work and said that it was an invalid key. Before coming here and contacting the OP, however, I tried again just because why not, and it all seemed to work just fine this time around. The time between attempts was just a few seconds, so I am actually not sure of what happened, but it seems to be working now. I will update this with more if anything else comes up.
Hi, it would be helpful to know which key you bought. When i bought my Win 10 pro key, i was upgrading from Win 10 home. And directly putting the product key did not work for me as well. I had to follow some extra steps from Microsoft (using a generic Win 10 pro key at first, I even had to disconnect my internet for the generic key to work), then restarting and updating the product key after.
Anyway, just an update from my previous feedback, I've installed all latest updates and it all worked fine. Installing Docker for Windows and integrating it to WSL also worked perfectly (its the only reason I wanted to upgrade to Win 10 Pro)
Yeah, I actually had a similar setup to you. The installation that I had on the computer at the time was that of Windows 10 Home after having upgraded a few years back from Windows 8. Unlike you, however, when I tried using the Windows 10 Pro key and it didn't work, I did not actually have to do anything other than just try again a few seconds later in order to make it work. I haven't really done anything other than regular and casual development use of the computer, but I do not see any reason for it to not work, so all is good over here. I will make sure that all the updates go through next time I use that computer in order to confirm that the installation is, indeed, seen as genuine from Microsoft's side. I'll post back if anything changes.