Apple only sells the current version.
You have to purchase on a mac running at least Big Sur (at the moment; this may change).
Here is how it can work: use a friend's computer running Big Sur, Monterey or Ventura to make the purchase using your Apple id. Delete the application from your friend's computer.
Back in your High Sierra mac, login to the App Store with your id, click "Purchased". Final Cut Pro will appear. Click the Download button. You will see an alert that the current version is not compatible, and be asked if you want the most recent version that works in your system. Confirm.