Hi, BloxxyVidsYT. I'm Robinson, and I’m happy to help you today.
When your dad upgraded from Windows 7 to Windows 10 the new Windows license for Windows 10 was attached to his Microsoft account.
To be able to activate Windows you will need to sign in using the same Microsoft account that was used on the computer during the upgrade from Windows 7 and run the activation troubleshooter and select the option "I changed hardware on this device recently".
1-Select the Start button, then select Settings > Update & Security > Activation > Troubleshoot . The troubleshooter will show Windows can’t be activated on your device.
2-Select "I changed hardware on this device recently", then select Next.
3-Enter your connected Microsoft account and password, then select Sign in. The troubleshooter will only work with the connected Microsoft account.
4-From the list of devices that are linked to your Microsoft account, select the check box next to "This is the device I’m using right now".
5-Select Activate.
I hope this information helps. If you have any questions, please let me know and I'll be glad to assist you further.