Windows 10: Windows 10 Pro key wont activate

Hello!

My name is Elmar and i recently bought a Windows 10 pro key, i am currently running 'Windows 10 pro EN 64Bit' without the activation key.
When i try to activate the key i get the error message 'The last product key you entered can't be used on this copy of Windows. 0x803FA067
I have looked up the error code but couldn't find any solutions, i have also tried the Troubleshoot.

:)

 

Windows 10 Activation (Insider)

I bought Windows 7 Pro OEM key from G2Play.net for 12€ to upgrade to Windows 10 Pro - worked like a charm but be advised if you buy a OEM key you can not download the Win 7 iso from microsoft and have to get it elsewhere.

 

Windows 8.1 pro + WMC key activation

This worked for me last time with a Win 8 key.

Install Windows 8.1 and enter KMS key during setup. Once in windows change product key to my Windows 8 key. Go to add features and enter my WMC add-on key. Drink a beer to a job well done.

 

Windows 8.1 pro + WMC key activation

So it will be fine installing 8.1 pro with a KMS key, then switch to my 8 pro key and it will activate ok?

Maybe a daft question but why can't you just install 8.1 pro with the 8 pro key, or is it different keys for 8 and 8.1? seems silly when 8.1 is a free update anyway.