Try using the generic product key to initiate the Office version change, then sign in with the Microsoft account you used to purchase Office to complete the activation.
Disconnect from the internet
Go to Settings > System > Activation. Click "Change product key," enter "VK7JG-NPHTM-C97JM-9MPGT-3V66T," and hit activate.
Reconnect to the internet and make sure you’re signed in with the Microsoft account you used for the purchase.
Restart your computer and check.