Get rid of colons in add parm

           string commandtext = "Insert into mpcst.metadata(BOUNDARY) values (:BOUNDARY)";

            cmd.Parameters.Add("BOUNDARY", OracleDbType.Varchar2).Value = DropDownList1.SelectedValue.ToString();

use the COLON only to signify the BIND variable in the Insert statement VAlues section

example

Imports System.Xml.Linq.XElement
  
    Public Shared Sub updateUnitsActiveFlag(ByVal decQuantity As Decimal)
      ' Insert Quantity into new row Units table'
      Dim OraConnStr As String = ConfigurationManager.ConnectionStrings("{YourOraConnStrName}").ConnectionString
      Try
        Dim SQL =
        <SQL>
        INSERT INTO {YOURSCHEMANAME}.UNITS
          (UNITS_SEQ, QUANTITY, DATE_CLOSED)
        VALUES
          (UNIT_SEQ.NextVal, :BindVarQuantity, :DateClosed)
        </SQL>
        Using conn As New OracleConnection(OraConnStr)
          Using cmd As New OracleCommand(SQL.Value, conn)
            cmd.Parameters.Clear()
            cmd.Parameters.Add("BindVarQuantity", OracleDbType.Decimal, decQuantity, ParameterDirection.Input)
            cmd.Parameters.Add("DateClosed", OracleDbType.Date, dateDateClosed, ParameterDirection.Input)
            conn.Open()
            cmd.ExecuteNonQuery()
          End Using
        End Using
      Catch ex As Exception
' you exception handling here
      End Try
  End Sub