https://www.canada.ca/en/revenue-agency/services/e-services/filing-information-retu[…]t4-t5-other-ty...

T5 - Account number
- Required, 15 alphanumeric, 9 digits RZ 4 digits, example 000000000RZ0000

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.

Sorry this is correct  link.... 

https://www.canada.ca/en/revenue-agency/services/e-services/filing-information-returns-electronicall...

Thanks for the update.

I sorted it out after 2 hours on the phone with CRA.

I needed an RZ0002 number, as the original RZ0001 number was associated with T5018 filings many years ago.  

Thanks for this @Sharon yes I mentioned it in some other post this morning, unfortunately we canot be of much help when it comes to finding the right account of our customers