"Final Cut Pro can’t be installed on “Macintosh HD” because macOS version 11.5.1 or later is required." but i have catalina osx 10.15.7

Your OS cannot run the software. Apple only sells the current version of the software. If you want an older version you first need to purchase the version Apple sells, which can only be done on a more recent OS. If you can make the purchase using your Apple ID you would then be able to download an older version.

The message you are getting is correct.

In Catalina you can only run an earlier version (perhaps 10.5.4, I can’t confirm right now).

Are you trying to run the trial, or the full version?

I am afraid that for the trial you can only get the current, so no luck on Catalina.

If it is the full version and you have purchased FCP already, click on your account at the bottom left of the App Store to see your purchases; click the Download button and you will be told that the current version is not compatible and asked if you want the most recent version that works in your OS.

If you are trying to purchase, you will have to do it in a mac running at least Big Sur, and using YOUR Apple id; then in your Catalina mac you can do as above described.

Hello,

maybe try to install on a different storage or Update your Mac

To Update your Mac:

Update macOS on Mac - Apple Support (MY)

Hope I Helped

Hi. I know, i don't use an mac, but i would say that you should try installing it on another storage.