Dear,
Are you not getting automatic creation of serial number at the time of GRN?
Please check the steps :
1) Did you assign the serial number profile in material master under tab "Plant data/stor.2" in "general plant parameters area"? as per valuation type as per your value category in material master.
2) When you do MIGO system creates the serial number and equipment (Prerequisites: If you have assigned the serial number profile in serializing procedure)
Regards,
pardhu