A family of Microsoft word processing software products for creating web, email, and print documents.
Sounds like you didn't install Office updates and/or did not activate the product.
Download a copy of the current version of AutoUpdate from this link, install and then run AutoUpdate to bring your installation to current update level, then restart your Mac. If the updater does not open automatically after download, double-click the downloaded .pkg file. Click this link to download the updater. The update is free.
https://go.microsoft.com/fwlink/p/?linkid=830196
Restart your Mac, then activate Office:
ACTIVATE MICROSOFT OFFICE
Stand-alone perpetual license:
Activate Office for Mac - Microsoft Support
-or-
MS 365 subscriber:
Activate Office - Microsoft Support