restart steam or launch it from your desktop shortcut. Mine was bought directly from steam and the play button was blacked out. i launched it from the shortcut it created on my desktop and it went through
I specifically said third party.
If you bought it from steam you're fine and those fixes may work. If you bought it from a third party like Green Man Gaming there is nothing you can do. I have uninstalled and reinstalled the game 3 times, restarted my computer and steam more times than I can count and there are thousands of other posts like this on reddit, steam forums, and social media. There is some problem with steam where it is not verifying and unlocking third party purchases.