Content Manager: Always Getting "Race Canceled" - Why?

I'm new at using Content Manager (full version). I'm trying to run a race against the AI and have been unsuccessful so far. The message at screen lower-left shows the track being loaded, then cars one-by-one --- but it stops then and I get a screen saying "Race Canceled".

Can someone explain why I can't get a race against AI to work for me? Is there a setting that I need to adjust somewhere?

FWIW - I'm running in WMR (VR) and practicing alone on the same track(s) works fine.

Thanks!