Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Yes, the problem keeps returning again and again.
It seems the problem emerged after they rebranded it from Office 365 to Microsoft 365.
The rebranding was just that: a new name. It won't affect how Office programs work.
Which specific type of subscription license are you running? Home user license such as Microsoft 365 Family? Or a business license?
Since something is broken, you may have to remove all traces of Microsoft Office applications, for example by using the Uninstall support tool (see Option 2 in the article at https://support.microsoft.com/en-us/office/uninstall-office-from-a-pc-9dd49b83-264a-477a-8fcc-2fdf5dbf61d8). Make sure that Windows is up to date. Then reinstall Office. Of course, if you are running a business version, your organization may not allow you to do this yourself.