This is caused when the client isn't linked to your agent profile on the ATO portal, or the ABN entered for the client has been cancelled, or the ABN is incorrect.
Follow the below:
Open the ATO portal and confirm that the client has been added to your agent number. If the client is already listed, remove them and then re‑add them.
Add the ABN in the ATO portal if an ABN is being used in the APS return.
Verify that the TFN and ABN match the details shown in the portal.