A family of Microsoft word processing software products for creating web, email, and print documents.
Sounds like you had a license for an earlier version of Office, but you're trying to activate a newer version of the software with your license that is still good for an older version.
Office 2019 licenses work only with Office 2019 software.
Office 2021, 2024 and Office 365 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 tell us what version of Mac OS you have and which license you purchased, we can refer you to the correct installer so you can continue to use Microsoft Office.