Final Cut Pro for High Sierra?

You are still not saying what is your mac model.

It is very likely that it can be updated to Catalina (10.15) and soon Big Sur (11.0) and run the current version of FCP, but you have to tell us what it is.

Go to the Apple menu and choose About this Mac... it will tell you not just "MacBook" but something like "MacBook Pro (13inch, early 2014)" or something like. Tell us.

Your mac is more than capable of running the current OS, Catalina, and the coming Big Sur.

Unless you need to continue to use some old software that does not run in Catalina, you should probably upgrade.

Don't do any major upgrade like this without a backup, though - that is usually when Murphy's law strikes...

So I'd strongly recommend you make sure to have an up to date Time Machine backup and, preferably, also a full bootable clone of your drive - so you can, if needed, boot from this and still use the older OS - and then upgrade to Catalina.

You can then download the trial version of FCP, and see how it works for you before paying for the full version.

The trial, by the way, is fully functional for a generous 90-day period.

Here is a suggestion: install Mojave 10.14.6, as Tom mentioned, but in an external SSD.

Download the FCP trial, which is version 10.4.10 and works in Mojave. Try iy out and see how it all works.

You can still restart to High Sierra in your internal drive, at least until you are confident that your software all works in Mojave (if that is the case; we can't say because we don't know what software you rely on).

If and when you feel confident, upgrade your internal drive to Mojave.

Or maybe you can tell us about the software you need... perhaps it can be updated to run on the newer versions of the OS?

One important thing to take into consideration: once Big Sur is released (which can be any day, now), High Sierra will probably not receive any security updates (barring any very serious issue that may arise). It is customary for Apple to keep security updates to two versions behind the current one.

If you can I would recommend you update to Mojave 10.1.46. It's the last OS that will work with 32-bit applications and content.

https://support.apple.com/en-us/HT210190

You can't buy an older version.

What mac exactly?

If your mac is recent enough (around 2012 or later), it can run Catalina and you can get the current version of FCP.

The current version sold in the App Store will not. It requires Mojave 10.14.6 and also has hardware requirements. Check the App Store to see if your system meets those.