Once you decide to go the legal route (or the 90-day trial), here is how to safely install FCP 10.5.4 or later.
If you need to re-download an older version like 10.5.4 because your macOS isn't compatible with the latest release, you can go to the section of the Mac App Store. According to the Apple Support Community Final Cut Pro 10.5.4 Free Download
Pirated versions frequently crash or fail during high-resolution exports. Once you decide to go the legal route