Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
This usually is not a lost license, but an activation/sign-in problem on that Mac. Microsoft 365 Personal can be installed on all your devices, but it can only stay signed in on five devices at the same time. If you go over that limit, Office can sign devices out automatically.
Please go to your Microsoft account subscription/services page and verify that the Microsoft 365 Personal subscription is active under the same email address used on that Mac.
account.microsoft.com/services
If you use several Macs and other devices, sign out of Office on old or unused devices from the Microsoft account installs/devices page. Don't exceed the 5-device limit.