This seems to be a very commonly encountered problem based on about three minutes hopping around sites from Google.
But the solution is very easy. I stumbled across it as while I’m preparing a test server running Windows 2019, I’m also multi tasking by installing a KMS server. That’s a story for another time.
Anyway. The installation of the product key fails with the error (0x80070490).
To get around it, just install the product key from the command line.
Open up the command line as an administrator and issue the following command. Of course, replace your product key with the MAC product key that you’ve obtained from Microsoft.
cscript c:\windows\system32\slmgr.vbs /ipk [your product key]
If only things could be straight forward.