This means exactly what it says. There is an Audio Unit installed that is not compatible with FCP X - probably installed by some audio application.
All you need to do is find it and trash it, or simply move it somewhere else.
See this knowledge base document from Apple about Audio Units: Finding Audio Units plug-ins on your computer
Notice that there are two different folders named "Library" - the general one, at the top level of the drive, and the user's, inside the home folder.
(Note: to access the Library folder inside your user folder, which is usually hidden, hold down option key and open the Go menu in the Finder)