You have not provided any alias to the SUM in sql query leading to no column named 'AmountPaid'. Modify to:
Dim strSQL6 As String = "SELECT Regno, SName, SUM(AmountPaid) as TotalAmountPaid, FeeType, Class, Section FROM FeeTrans where...
Post this, bind the new column name of dataset to your grid as:
.Cells("Amount_Paid") = dr("TotalAmountPaid")
P.S.: I also see that the syntax of adding new columns seems incorrect. Add method with two parameters tells to add column of a given name and type.
Refer: MSDN: DataColumnCollection.Add Method (String, Type)