I’m getting this error when attempting to reference the AccountUser subtable on another table (Activity).
The answer is, yes. A subtable can be used in a reference field on another table. Something was not in sync on my project. Refreshing the table schemas was not correcting the issue. So I undid all changes I made today and then reapplied them 1-by-1. Then I was able to successfully add a reference field to the subtable on another table.