The Creeping Grey: iTunes greyed out ALL of my songs on ALL of my playlists

All of these songs I bought on iTunes. I'm logged into iTunes now. Like I just added a song to my playlist and burned a CD, and it was fine. And it's not greyed out on my song list.

But right after I burned that CD, when I tried to add that exact same song to another playlist to burn, it and two others I added greyed out, and the computer burned only the first 9 songs, leaving out the 3 greyed ones.

In fact, now ALL of my playlists on the computer are now greyed out, even the one I just burned. IOW, first all 12 songs burned, the second time only 9 burned and 3 greyed out, and now ALL are greyed out, like the Grey Plague was quickly creeping up my playlists as I watched. This just now happened after I burned the first CD I mentioned above. And when I try to burn a playlist, iTunes says there are no songs on that playlist. Yet those same greyed out songs will play from the playlist, probably because they are NOT greyed out on my song list. But I can't burn any of them to CD. iTunes has always been glitchy, but this is a new one for me.

I'm running a 2011 iMac (it's been flawless its entire life), Sierra 10.12.6, iTunes 12.8.0. When it comes to choosing a device from Apple's drop-down menu "What device is this about?" Apple assumes - wrongly - that it has to be a 2012 or later and there is no choice to choose an earlier model. Control issues.