Hello,

If it it running Mojave as your equipment line shows, major system upgrade notifications and links are now in System Preferences under "Software Update," not in the Mac App Store as in High Sierra and older. This from my iMac running Mojave:

See: How to update the software on your Mac - Apple Support

You absolutely need a complete and current backup prior to upgrading.

Here is How to upgrade to macOS Catalina - Apple Support

Verify that your apps, your printer and scanner drivers, and the rest of your apps and tools are all 64-bit or that 64-bit updates are available, or that you have 64-bit alternatives. 32-bit apps are not compatible with Catalina. The Go64 tool can scan for 32-bit apps.

You will need to upgrade any version of Microsoft office prior to Office 2016 to newer, if you are using an old version. Office 2016 (soon deprecated), Office 2019 (current), or later (as available), or Office 365 subscription are required, or a migration to the free LibreOffice app, or to the Apple iWork apps Pages, Numbers, and Keynote (which can read and write many Office files), if you are using Microsoft Office documents.

I think I could help you,kodjo273.Catalina breaks support for 32 bit apps. If an app is not 64-bit, it won’t run in Catalina. So an app such as Microsoft office will have to be upgraded to work in Catalina, Or if you had Mojave, it may need to be updated. That is why the older version of Microsoft office you have now is no longer supported. You’ll have to buy an upgrade for it.Might I suggest open office or libre office as a low-cost alternative to that?

john Brine