Hi Omkar,
You can find the source of this error message by debugging.
before saving the sales order just put /h in the ok-code box and press enter and then save the sales order.
keep on pressing f5 or f6 and have patience, you will find the source of error message. else one more way to find out the source:
while debuging instead of pressing f5 or f6, double click on the message class declared in the abap program
and later on find the message where it has been used. find its code as 110 or 120 etc.
and then come back to program and search this message code.
you will find the source of your error message.
As an abaper i can suggest you that much only. Rest is in the hand of your ABAPer only.