Hi Anitha,
Try to check the Serial Number Profile.
Go to TCODE OIS2. In the serializing procedure, is the Procedure MMSL maintained with Serial Number Usage 02 (Optional) or 03 (Obligatory)?
Maybe it was maintained with 02, so that the system still enable user to save without assigning it with serial number.
Hope this can help.
Best Regards,
Hengky