Thanks QuaG for checking. I believe I have it working now. I thought I’d post the info in case it helps someone else.
Apparently that error can occur if you don’t have the map in question. In this case, I had not subscribed to the particular map in the Workshop section of Steam. After I tracked it down, subscribed, and retried, it worked. Why doesn’t Contagion download the map automatically like most Source-based games but instead shows a cryptic error? Your guess is as good as mine.