Use a friend's Mac or a secondary machine running a newer OS (like Big Sur, Monterey, or later). Log into the Mac App Store using YOUR Apple ID Purchase FCP:
Final Cut Pro 10.5.4 is the final build that fully supports macOS Catalina 10.15.7. Later versions (10.6 and above) require macOS Big Sur or newer. Final Cut Pro For Catalina 10.15.7 Download
Catalina 10.15.7 has known GPU driver regressions on older Intel Macs. Use a friend's Mac or a secondary machine