Thanks for the helpful answer - much more useful than the one I got from Apple technical support.
Here's a disturbing report:
I called Apple with this problem and the tech put me on hold while she checked for a solution.
She came back and told me there was no solution for this other than to upgrade to High Sierra.
I explained that, while we were amenable to upgrading, there were a number of practical reasons it was not feasible to do so on the spur of the moment just to fix Apple music.
But the tech was insistent that there was no known fix other than going to High Sierra.
I expressed skepticism, as this is an issue that has been reported by thousands of users more than once in the past, before High Sierra was released, and so it likely had a more mundane answer.
Nope. The tech said that this is the answer they were instructed to give in this case. Go to High Sierra, or she couldn't help me.
So I searched the web and found your answer in about 10 minutes, and it worked.
I actually pay for tech support so "Go search on the forums" isn't an acceptable result.
If there's a quick solution, and Apple techs don't know it, that's not satisfactory.
If Apple tech support does know the solution, but declines to offer it to those who haven't gone to High Sierra, that's worse.
It would be ok if they made at least a token effort to solve the problem.