My solution: I went into my apps folder, took FCPX and moved it to the trash. When I went to update my FCPX tonight (It's been at 10.2.1 for sooo long), I got the same message 'Incompatible Update.' The App Store was forcing me to upgrade to version 10.5.1, which would of course require me to also upgrade my OS to Mojave. I've been upgrading my old mid-2010 Mac Pro (new metal/RX580 GPU, amongst other things), and got it to High Sierra where I would like this machine to stay so that I can continue to also use CS6 for my photography work. I know this question was from a year ago and it's likely been resolved by the poster, one way or another, but I had the same situation and found a super simple solution that worked for me. Otherwise, follow Tom's advice If you can. I'm still on HS but now considering upgrading to Mojave mainly for the latest version of FCPX. I know it exists and I used to have the direct link, but access may be tied to Mac and Apple ID. If not, call Apple support and they may direct you to the appropriate download. From there, Apple does have FCPX10.4.6 (last version to work on High Sierra) on their servers and it may direct you to the proper version in App Store. Link in support document to older trial version.

I believe that this page is still active and will allow you to download FCPX 10.2.3 free trial, not current version. I'm rocking on a 2012 rMBP and really want a new Mac, at least dreams are free. 10k $$$ should get you going well, but for me, not going to happen today. You are sporting a pretty old Mac, so performance will suffer, especially with high resolution media.

How do i update my mac to 10.13 6