iTunes 12.6.2.20 will not let me turn on iCloud Music Library
I still buy CDs, and the only way I can get them onto my iCloud Music Library is via an old Mac running iTunes 12.6.2.20 and OS X Version 10.9.5. This is far from ideal, but that's the setup I have and until today I haven't had issues with it. This morning I burned a few CDs and noticed that they weren't appearing in my Library on my phone or MacBook. The iCloud icon in iTunes was showing as a dotted line, and permanently stuck on Waiting.
I searched around and tried various things, logging in and out of iCloud, my Apple Music account, turning Genius (which I have never used) on and off and so on. Now I'm worse off than when I started, because it will not let me switch iCloud Music Library on (by ticking the box in iTunes > Preferences > General). Every time I try to do this, it gives me the error message "Genius results can't be updated right now. An unknown error occurred (-9824)."
I have tried absolutely everything I can find to resolve this, but nothing seems to work. I can't even find any trace of error -9824 anywhere. If anyone has any suggestions, I'd be happy to hear them.