Missing or offline titles, effects, generators or media in FCPX

Why is it that before exporting my project in fcpx, I get this message: Missing or offline titles, effects, generators or media. if you
continue, these items will be replaced with missing-media projects.
Today after this message I just went a head and exported and there was
one or two missing clips. However, when I checked the timeline, those
clips were present. How can I make sure that nothing is missing when I
export my project and how do I get rid of this message?   Please help.  Thank you.

I have no idea what is causing your problem but whenever FCP behaves oddly for no obvious reason a good starting point for troubleshooting is to delete the preferences.
Hold down the Option and Command keys as you launch FCP until a window asking whether you want to delete the preferences appears. Click "Delete Preferences".
FCP will open with an empty interface so you will have to navigate to the project you were working on.
Restarting the Mac is also a good first strategy.

Thanks for that INBRO. I will try it. Cheers!

So after I delete fcpx preference files, do I just click on the fcpx purple files in the external hard drive to restore them into fcpx? Is that correct?

Yes, just double-click the Library file and it will open in FCP.

Thank you inbro. You're awesome! Cheers!