how to remove this error="data type mismatch in criteria expression"

Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click

        If Namount.Text = Nothing Then
            MsgBox("insert data first")

        Else

            Dim str, str1 As String

            str = "insert into customer values(" & Val(bill.Text) & ",'" & dtp.Value & "','" & cName.Text & "','" & Address.Text & "','" & paymenttype.Text & "'," & Val(phoneno.Text) & "," & Val(Tamount.Text) & "," & Val(Discount.Text) & "," & Val(Namount.Text) & ")"


            cmd = New OleDbCommand(str, cnn)
            dap = New OleDbDataAdapter(cmd)
            ds = New DataSet
            dap.Fill(ds)


            Dim i As Integer
            Dim Barcode, Item_name As String

            Dim Qty, item_rate, i_amt As String


            For i = 0 To DataGridView1.RowCount - 1
                Barcode = DataGridView1.Rows(i).Cells(0).Value
                Item_name = DataGridView1.Rows(i).Cells(1).Value
                Qty = DataGridView1.Rows(i).Cells(2).Value
                item_rate = DataGridView1.Rows(i).Cells(3).Value
                i_amt = DataGridView1.Rows(i).Cells(4).Value




                str1 = "insert into item values(" & bill.Text & ", " & Barcode & ", '" & Item_name & "'," & item_rate & "," & Qty & "," & i_amt & ")"

                cmd = New OleDbCommand(str1, cnn)
                dap = New OleDbDataAdapter(cmd)
                ds = New DataSet
                dap.Fill(ds)

            Next
            MessageBox.Show("Record Added")


            bill.Clear()
            paymenttype.ResetText()
            cName.Clear()
            Address.Clear()
            phoneno.Clear()
            DataGridView1.Rows.Clear()
            Tamount.Text = "00"
            Discount.Clear()
            Namount.Clear()
        End If