how to add desktop computer to my Microsoft 365 account when the system is not recognizing its serial number

Hi Todd Hinds,

Good Day. I hope everything's well.

In regards with your installation issue, you can try to sign-in through Microsoft Account inside Office (word, excel, outlook, etc.). Devices are automatically registered when you sign in with MS account. On Desktop PC, go to official installer https://www.office.com > Sign-in with the same Microsoft 365 account you used on your 2 devices (laptops) < once Signed-in, click Install Office > Microsoft 365 apps. Run the installer and complete setup. This automatically registers the desktop to your Microsoft 365 subscription - no serial number required.

Second, Check also if your current subscription is active. Go to https://account.microsoft.com/services and check if it is listed as "Active".

Note: recommended to uninstall any old or preinstalled Office versions. Uninstall all entries named Office or Microsoft 365. Reboot your PC and reinstall Office using the first procedure.

I hope this helps.

Regards,
Jhun

@Todd Hinds What are you trying to do? If you are trying to install Office for your son, the correct way to do that is to share your subscription with him. You can only do that if your subscription is Microsoft 365 Family (or Microsoft 365 Premium).