just finished building my new pc and my activation key is linked on microsoft account. but when when selecting the device that the key is on it wont let me reactivate
Go to your activation settings, and select "Change Product Key". Enter the product key that came with your copy of windows, and then try to re-activate again.
Go to your activation settings, and select "Change Product Key". Enter the product key that came with your copy of windows, and then try to re-activate again.