LodgeiT

 If some other agent will add the client by TFN to their list in ATO portal it will break the existing relationship with any currently nominated agents, so please let the user double check the relationship with the client still exists.

Another reason of why the user gets this validation message could be the following permissions (marked with yellow/red) on ATO portal, so please let the user check it ticked for specified client as well:

Please let us know if it helped. Otherwise, we will have to raise the incident with ATO.