CS Error: Cannot generate the next number for the sequence Custome DAC ?

I got CS Error: Cannot generate the next number for the  sequence on Custome DAC and Custome New Screen 

 

This My DAC

 

    public static class MyNumberingSequence
    {
        public const string SequenceID = "BUYINGDOC"; // New sequence ID
        public class sequenceID : PX.Data.BQL.BqlString.Constant<sequenceID>
        {
            public sequenceID() : base(SequenceID) { }
        }
    }


1. Why i got erorr CS Error: Cannot generate the next number for the  sequence ?
2. Why I got  <SELECT> not <NEW> , can u help for change the default value to  <NEW> like reciept secreen entry ?
 

Can you help me sense ?
Thanks