Not able to add Serial Number in /SCWM/PRDO

Hello Experts

   I have a outbound delivery for a product that is serialized in ECC & eWM.

I am trying to add Serial Number under Serial Number tab and I am getting following message.

I have checked product is serialized and in outbound delivery field Serial No. Reqm = C. 

Serial numbers cannot be changed

Message no. /SCWM/DLV_SERIAL303

Thanks

Ankit