Please check the following -
a. That the client is in your ATO Portal & you have permission to lodge on their behalf
b. That the ABN & TFN are recorded correctly in LodgeiT
c. That the ABN has not been cancelled. If it has, please remove it from LodgeiT.
d. Possibly there is a Branch Code. If so, attempt to replace 001 with 002 at Settings -> General.