You were running Microsoft Office 2011 or earlier, and have now upgraded to Catalina macOS 10.15 or Big Sur macOS 11.
Catalina and later versions do not support older 32-bit apps like Office 2011 and earlier.
To use Office on Catalina and later, you will need to acquire newer version.
The Office 2019 (purchase) and Office 365 (subscription) products are for sale by Microsoft. (The older Office 2016 will also work, but that is no longer for sale.) You can purchase and acquire these Office downloads from Microsoft, or through the app store.
Or you can migrate to LibreOffice, which is free. See: https://www.libreoffice.org/
Or the App,e iWork apps can read and write many Office documents, amd are free.