Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Hi,
Your Mac still has leftover Microsoft 365 trial license data, which conflicts with your Office 2021 activation. to fix this:
Remove old licenses using Microsoft’s License Removal Tool.
Reinstall Office 2021 from your Microsoft account and sign in with the correct account.
Here’s the official tool link:
How to remove Office license files on a Mac - Microsoft Support
I hope this helps!