If you use iZotope audio plugins in Final Cut Pro X and you're installing them on macOS Catalina (currently 10.15.1), you may run into an authentication issue, warning that the plugins "did not pass the Apple AU validation." iZotope has a help article detailing how to manually install RX 7 that you can follow, but in some cases you still have this troublesome popup in FCPX.
If you are running into this window, here's a quick fix that worked for me:
1. Navigate to the Preferences folder in your User Directory
In the Finder, select Go in the Menu Bar while holding Option and your User Library will appear in the list.
2. Find the file called "com.apple.audio.InfoHelper.plist" and move it to the trash.
3. Restart your computer and launch Final Cut Pro X.
You may run into 30-40 windows warning you about unsigned iZotope plugins, but if you open System Preferences and navigate to the Security and Privacy tab, you can select "Allow Anyway" after you hit "Cancel" on each warning window. It's a hassle, but you can get rid of these in a minute or two of bouncing back and forth between the two. Eventually you will start to see "Open" on the warning windows. Once all of the windows have been approved, FCPX will open normally.
I hope iZotope will fix their installation process to be fully compatible with macOS Catalina, but until then, this workaround should take care of any issues.