If when you try to activate your Windows, you receive the following message, The product key you typed cannot be used to activate Windows on this computer, then try the following:

1] Confirm that your product key has been bought from a genuine reseller or source

2] Check if you are using the correct key (Retail / OEM / Volume / etc) for your Windows.

3] Check if the key is meant for that particular Windows edition. (Ultimate / Pro / Home / etc)

4] To analyze and solve problems with Activations and Validation, run the MGADiag or Microsoft Genuine Advantage Diagnostic Tool.

5] This post may help you Troubleshoot Windows Activation States.