If what you have (most likely) is Office 2011, the only free method to continue using it is to restore the backup you made before installing Catalina.
Office 2011 is 32 bit software and will not run in Catalina. No 32 bit apps will. That, and MS will never update Office 2011. All support for it was dropped over a year ago.
If you have other apps that require Catalina, then you have no choice but to replace your old version of Office.
- Purchase the one-time cost, perpetual license Office 2019.
- Purchase the monthly/yearly subscription Office 365.
- Use Apple's free similar apps, Pages, Numbers and Keynote (they're not very good at correctly reading in Office files).
- Try a free alternative suite such as LibreOffice or OpenOffice. Note that neither of these has a replacement for Outlook.