Product key of windows 7 is not accepted for windows 10

I've just upgraded my activated Windows 7 Professional to Windows 10 Pro.

I upgraded, rather than making a fresh install.

During installation I was asked for the product key, it wasn't accepted, so I proceeded without the key. Now when I try to activate I get this error:

You are running Windows 10 Pro. The Product key you entered cannot be used to activate this edition.
Error Code: 0xc004f210

I looked into similar questions like this and this, but the difference is that I upgraded (rather than doing a fresh install) from Win 7 professional to Win 10 pro. I don't mind reinstalling win 7 and doing it all from scratch again. But I was wondering if there was an easier way.

The error code 0xC004F210 means that the Windows 7 key is a different SKU

//
// MessageId: SL_E_EDITION_MISMATCHED
//
// MessageText:
//
// The Software Licensing Service reported that the license edition does match the computer edition.
//
#define SL_E_EDITION_MISMATCHED          ((HRESULT)0xC004F210L)

During setup of Windows 10, simply skip the key. Windows 10 will detect that you already did the upgrade and shows Windows 10 as activated after setup is finished.