my iMac didn't update

Microsoft only supports the the latest macOS versions, which are Sonoma v14; Ventura v13; and Monterey v12. If you are running macOS 10.15 Catalina as shown in your equipment profile, that is the problem—they dropped support some time ago.

This is the MS support document on the subject:

https://support.microsoft.com/en-us/office/upgrade-macos-to-continue-receiving-microsoft-365-and-office-for-mac-updates-16b8414f-08ec-4b24-8c91-10a918f649f8

If your iMac is too old to update toat least Monterey, you ned a new computer or try an alternative like LibreOffice

In addition to LibreOffice, don't forget you have apps already on your Mac that can read and write Microsoft formats. Pages, Numbers, and Keynote are all included with Macs, and do a good job of cross platform communication, when needed.