Are you using Affinity? The only mention I can find of using an Agent Services account in the KB is talking about Affinity users, where you link your ASA to your top level Affinity dashboard and then you can use it to submit returns for any connected QuickFile accounts for which you are authorised to act. If you’re not using Affinity (you’re just a regular login on the client’s standalone QuickFile account) then it may be that the client needs to make the connection themselves with their own Government Gateway login before you can use that connection to submit the return.
Note: I don’t work for QuickFile, only @QFSupport can confirm for definite.