final cut pro x for mojave 10.14.6 PLEASE!!!!!!

The current version 10.5 or later will not work with Mojave I'm not sure, but I think somewhere around 10.4.6 is the last version for that OS.

As Apple only sells the current version of software, you have to find a way to buy that before you can get the older version. You can either can an external SSD and download Big Sur and install it on that drive. You can boot into the drive and make the purchase in the App Store using your Apple ID. Once you've done that you can then go back to Mojave and, signing into the App Store with the Apple ID used to purchase the application, download the appropriate one for your system. Another way is to find a friend with a newer OS, late Catalina or Big Sur, and using your Apple ID sign into the App Store on their system and make the purchase there before getting the appropriate version on your own system.

It's unclear, have you purchased the application in the past, or are you trying to buy it? Apple only sells the current version. You would need to purchase that first before you can download an older version.