Try with this...
select Product_Material_Master.Product_Name, SUM(GR_Process_Outward.Qty),GR_Process_Outward.Material_Code
from Product_Material_Master,GR_Process_Outward
where Product_Material_Master.Product_Code=GR_Process_Outward.Product_Code
AND Receipt_No=5
GROUP BY Product_Name,Material_Code
whenever you used the Group function definately you can call all the columns whatever you select in select query except that aggregate functions like( sum, avg...)