
Office 2019 licenses work only with the corresponding software which runs only in specific versions of Mac OSX.
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