‘project’ has missing or offline titles, effects or generators etc

[Tim Wilde]

I ended up at this thread with the same error message but in my case the cause was:

After sweetening in LPX I imported the resulting audio file into FCPX and aligned it with the existing video and audio. I then detached the original audio so I could disable it.

The next day, when I booted up and started using FCPX it started issuing the “missing…” warning every time I tried sharing or sending to Compressor. There was an exclamation point on the project icon. When I made a snapshot of the project as recommended in this thread, the snapshot also had the exclamation point.

The solution turned out to be simple: delete the disabled old audio track.