Not entirely clear if OP was trying to book an itin that consisted of all DL metal or not. The DL Amex 15% discount on awards only applies to award itins that are 100% DL metal. Also, getting #100503 error has nothing to do with whether or not 15% discount applies. That would be some other issue (it should still let you book awards that don't qualify for discount without the 15% off). I believe the #100503 error is shown for award booking where DL is seeing phantom partner inventory availability.