Basically, what David McKinlay stated.
Even if you upgraded to Catalina at this point, once Microsoft shifts their minimum supported operating system to Big Sur in Fall 2022, software updates for Office subscriptions, or the single-purchase Office 2021 for Mac would end on Catalina — because it would have dropped off Microsoft's supported platform list.
What you might consider is a replacement Office suite for MS Office, and a good choice would be SoftMaker's Office 2021 solution. Works on macOS all the way back to High Sierra, with Windows and Linux products as well. Although there is a free trial, it is not a free product and is very likely easier to adapt to than the alternative, and free LibreOffice Suite. The other bonus is that neither of the linked alternative products would require you to upgrade from Mojave.