family tree builder not compatible with OS catalina

You don't, as Catalina only supports 64-bit applications. Ask the Family Tree developer about their Catalina support, and Big Sur support going forward.

Alternatively, and depending on your MacBook Pro storage and RAM configuration, you could run a Parallels's, or VMware virtual machine with an earlier version of macOS as the guest operating system — with Family Tree builder on that guest.

Another approach would be to run this application on another Mac with older operating system, or as a last resort, revert to a prior version of macOS from Catalina by booting into Recovery and restoring from the Time Machine backup that you ran just before upgrading to Catalina.