Finalcut Pro X won't work on Mojave - Prompting me to spend $300 for the software!

I recently upgraded my desktop Mac to Mojave.

Now, FinalCut Pro X won't work (it's x'd out in my doc). I clicked on it and got this message:

The version of Final Cut Pro installed on this Mac is not compatible with macOS Mojave. You have “Final Cut Pro” 10.2.3. Download version 10.4.3 from the App Store.

But when I go to the App store, there is no "update." It is promoting me to spend $300 on the product! I'm very confused! Do I need to re-buy the product now that I'm on Mojave? And what about the files and videos that I already have?

Please help!