ADO.Net c# 0x80131904 must declare scalar variable/function

I'm trying to use a function via a SQL connection I've done everywhere else in my application (only here it give the error, not the rest of the application). When i searched for what that error code meant the responses i found say it's an error when one can't connect to SQL server? but it doesn't give a solution.

here is my c# code

 SqlConnection connection = Database.GetConnection();

                DataTable dt = new DataTable("CRC");
                SqlCommand cmd = new SqlCommand("SELECT dbo.CalcRentalCharge(@RentalStartDateTime,@RentalEndDateTime,@CarTypeID)", connection);
            try
        {
            connection.Open();
            using (SqlDataReader dr = cmd.ExecuteReader())
            {
                cmd.CommandType = CommandType.Text;
                cmd.Parameters.Add("@RentalStartDateTimetext", SqlDbType.DateTime).Value = RentalStartDateTimeBox.Text;
                cmd.Parameters.Add("@RentalEndDateTimetext", SqlDbType.DateTime).Value = RentalEndDateTimeBox.Text;
                cmd.Parameters.Add("@CarTypeIDtext", SqlDbType.Int).Value = CarTypeID.Text;

                connection.Open();
                Decimal rentalChange = (Decimal)cmd.ExecuteScalar();
                connection.Close();
                MessageBox.Show("The rental change is: " + rentalChange.ToString());

                if (dr.HasRows)
                {
                    dt.Load(dr);
                    dataGridView1.DataSource = dt;
                }
            }
            connection.Close();

Can you help me get this FUNCTION to work?

Don't use cmd.ExecuteReader() before adding parameter to command object. It gives error, add parameter to command and then cmd.execureReader()