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

Unknown Error

An unknown error has occurred. The error code is: 0xD0000019

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.