Please follow the instructions below to reset the AU validation.


Go to the Finder, click on the "Go" menu, click on "Go to Folder," and enter the following file path:

 

/Library/Audio/Plug-Ins/Components


Find the Plugin Alliance plugins and throw them away.  Then launch FCPX, and then quit FCPX.


Next, download the required installers.


Then launch FCPX and it will rescan.  Also, if you own Logic, you can go to Logic > Preference > Plug-in Manager and select "reset and rescan".