Thank you, however:

1. the CRA link you provided does not work

2. I still get the same error message, "Filer account number is missing or invalid for this type of return"

In this case, this happens whether I try to file from within Profile, using the client's RZ number and a Web Access Code, or if I try to use Represent a Client. 

The Filer account number has been entered; the file has been saved with this information.