Need a copy of FCPX compatible with High Sierra

Yes, you're in some sort of OSX vs FCPX version loop. 10.4 works with HS as does 10.4.5. (I keep all prior versions when updating just so I don't have to beat my head against the wall doing what you're trying to do now.) Some plugins will only work with the newer versions and some may only work with the earlier versions. This can be quite maddening. One of the Macs here is 2009 Mac pro 4.1 with firmware tweak to make it appear as a 5.1 and IS running HS and could conceivably load Mojave. Contact me when you get tired of being bounced around advice wise here.
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BTW I, can NEVER find my purchased apps when digging through the app store except from one dinosaur computer I have to revive just for that purpose. It's quicker doing that than attempting to figure out why it can't be done logically or normally, like in this guys case here.

I had a similar need but different circumstances and found a solution.

I had bought FCP previously in an older os before High Sierra. I just upgraded to HS from El Capitan and wanted to update FCp to the latest version that is supported in HS. In the App store, When I clicked 'Update,' a pop up appeared saying, 'Download an older version of FCP? The current version requires OS X 10.15.6 or later, but you can download the last compatible version.'

When I click 'Download,' the pop up would disappear, then immediately the same popup would appear again. I was stuck in a loop and it never actually downloaded anything. After reading in somewhere, I put my old version of FCP in the trash, quit the app store, then opened it up again. This time the button for FCP said, 'Install,' instead of 'Update.' I clicked it and the same popup appeared about downloading the last compatible version. When I clicked, 'Download,' this time it worked and it installed FCP 10.4.6.

If you haven't already tried, put your old version of FCP in the trash and try again from the App Store. I did this for Compressor and Motion too