final cut pro x on high sierra

You would need to purchase the application on a current system. Then you'll be able to download the applicable version for High Sierra on your machine. Perhaps you have a friend who's running 10.14.6 or 10.15.x who could help you do this.

Yes, but it is not straightforward if you haven't yet bought the application.

Apple only sells the current version (10.4.8 at the moment), and that needs at least Mojave 10.14.6.

I am not sure which version is latest that can run in High Sierra - maybe 10.3.4, or one of the early 10.4.*.

If you have temporary access to a friend's mac running at least 10.14.6, you can purchase the application - make absolutely sure to use YOUR Apple Id to make the purchase. Then delete the application from your friend's mac.

Go back to your mac, login to the App Store, and go to Purchased.

Click on "Download" next to Final Cut Pro. You will be advised that the current version is not compatible with High Sierra, and be offered the choice to get the most recent version that is.

@Luis

You can run up to 10.4.6 on High Sierra. I am! Speaking of versions, I am amazed that pre-10.4 didn't have the advanced color correction panels. Just the color board for all pre-10.4 versions? I myself have made use of all 4 and have done wonders with them, fixing problems with my main movie, which I could not have done with only the color board. And no keyframe animations of color controls before 10.4? Yep, I've used that too, but copying and pasting keyframes on the Color Curves doesn't work right. The comparison viewer, which was added with 10.4.4, is handy. And 10.4.6 appears to be the last of the online help that lets you stay in a single window with full navigational capabilities. I don't see why the new format is better. I think it's worse.