In the row with sum(Field3), you have to define it alias or sum(Field3) as Field3.
ADD is for partial loads. Maybe you are doing a partial load, but you might be looking to concatenate the tables. It that's the case you should use the function CONCATENATE.
If you have a sum in the script, the other fields in the load have to be included in a group by:
sum(Field3) as Field3
Group by Field1,Field2;
That's all I can see for now. I hope that helps.