Microsoft 365 issues after updating Mac to macOSCatalina Version 10.15.7

Your question is unclear.

Microsoft no longer supports applications running on Catalina. The oldest version of macOS supported is macOS 13 Ventura.

MS 365 is a subscription service. If you have MS 365, you already have (or had) a subscription.

What Microsoft used to call Microsoft Office is now Microsoft 365.

There are three basic forms of it:

• Microsoft 365 – subscription service with access to desktop and mobile applications
• Microsoft Office Home and Student – one-time purchase version with fewer features
• Microsoft 365 on the Web – versions of Word, Excel, PowerPoint that run in a Web browser (not natively on your computer) and that are free to use – for now

WIth either of the first two, Microsoft only supports the three most recent versions of macOS – currently: Ventura, Sonoma, Sequoia. If you have an older version of macOS and cannot or will not update it, you can continue to use any 365 or Office product you already have (cross your fingers and hope not to have to do a "clean' reinstall), but you will not get any updates, including security updates.

Microsoft Support – Upgrade macOS to continue receiving Microsoft 365 and Office for Mac updates

Catalina fell off the list of the "most recent three" in October 2022, when Apple released Ventura, so I would guess that you have not been receiving Office or 365 updates for some time.