You receiving the following error message on your Mac after attempting to activate Microsoft 365 apps:

Unable to Activate Office

We were unable to activate Office. Please try again later. You can view documents for free in the meantime if needed.

Unknown

1) Close the app and re-open it. Then, try activating again.

2) If that doesn't work, de-authorize previous Microsoft Office installations from your old devices by following the directions at https://support.microsoft.com/en-us/office/deactivate-an-installation-of-office-767e3560-96ed-4b1c-806d-2fe01c529b1b.

NOTE: It can take up to 72 hours for Office to detect that you signed out remotely.

Outcome

You will be able to activate your Office products.