Tom said
"not unless you have an OS and a software version 10.2.3 or earlier."
You still may be able to get FCPX ver 10.2.3 and the OS to run it, which is Yosemite.
As you have a Mac currently running 10.13.6 then this may suggest that you cannot upgrade to Mojave.
If this is true, then you have A Mac that was built before 2012 which would mean that it can run Yosemite.
If your Mac was built before this 2012 date, then you can install Yosemite on an external drive.
Get Yosemite here...https://support.apple.com/en-us/HT210717
It is likely to be version 10.10.5
Google how to instal OS on an external drive, very straightforward. Just be sure not to erase your internal drive.
When you have done this, you will need to boot to Yosemite from this external drive, again Google it, easy.
Whilst in Yosemite go to the App Store and sign in with your Apple ID, same one that enabled you to download 10.3.4
Go to your Purchased page and you will be able to download the version of FCPX that is compatible with Yosemite, which will be version 10.2.3
You should now have Yosemite and FCPX 10.2.3 and be able to do what you wanted to do.
