I need Final Cut Pro version 10.6.5

Sorry if my previous post was too terse. I was inside a bus at the time.

10.6.5 is the last version that works in Big Sur.

Log in to your Apple id in a mac running Big Sur, and click on your account at the bottom left of the App Store window to see your purchases.

Click the download icon next to FCP (*)

You will be advised that the current version is not compatible with the OS and asked if you want the most recent compatible version.

(*) this only works in your account’s list of purchases and not by searching the App Store at large for FCP.

Also note that this could be done using a friend’s mac running Big Sur or by installing Big Sur on an external drive and booting from it (if you have a compatible mac).

Run Big Sur and download from your purchases

Luis described the only way it can be done through the App Store. Off you can install Big Sur on an external drive you might be able to do for yourself.

Please give details of the computer and OS you’re using.

Which computer exactly? What year? What OS? The current OS will not run that software?

I'm in the same boat, and you'll be hard pressed to find anyone with an archived copy. And if you upgrade to Sequoia, it simply won't work at all, no matter what FCP/Motion version you have, period.