I think you try to bind data to data grid using sqldatasource from design side as well as using c# code from code behind side..
My Friend you have to do this for solving your error..
1) You need to chose one way to bind the grid
2) if it is from code behind means using c# code then remove the datasourceid property from grid view from design view of grid
<asp:gridview id="cartData" runat="server" autogeneratecolumns="False"
<asp:gridview id="cartData" runat="server" autogeneratecolumns="False">
3) if you use a design side binding to your grid then you have to remove the c# code to bind the grid.
Because you have to use only one method of biding, both at a same time is not permissible..