The "protected and unavailable" message means that the songs are copyright (DRM) protected and cannot be used in iMovie. Songs that were purchased before 2009 carried DRM protection. iTunes later dropped DRM protection for songs purchased after 2009. Also, songs in Apple Music are DRM protected. I'm not sure, but I think that if you download music from Apple Music into iTunes that might remove the DRM protection.
If you are not using Apple music, what you can do is subscribe to the iTunes Match program, for about $24.99, and that will allow you to download from the cloud DRM free copies of your songs, provided that iTunes still sells the song. For more info open the iTunes site, click on "Store" at the top of the screen, and scroll all the way down to the bottom of the window. You will see "iTunes Match" in the small print items at the bottom. Click on that and you will see details about the program.
Another option would be to repurchase the song from iTunes. That might make sense if you intend to use only a few songs in an iMovie project.
-- Rich