Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
It sounds like there is a mismatch between the software you installed and the license you're using to activate it.
You might have previously installed or had a license for Office 2019.
Office 2019 licenses work only with Office 2019 software.
Office 2021, 2024 and Office 265 licenses work only with the corresponding software.
To find out which software is currently installed, in Finder, go to the Applications folder. Right-click on Microsoft Excel.app and choose Get Info from the menu. The version is displayed near the top of the resulting dialog.
16.78.3 is the software for 2019 licenses
Anything 16.79 or higher can not be activated with a 2019 license.
If you have the wrong version of the software installed, here's how to remove it:
Uninstall Office for Mac - Microsoft Support
You might also remove all previous sign-ins using this tool. Restart your Mac after using this tool:
How to remove Office license files on a Mac - Microsoft Support
For Office 2019 licenses here is the installer for 16.78.3