Credit note issue

I have issued credit note to my unregistered dealer against sales return.

as the sale had come under b2cs head so no invoice number had been entered earlier in b2cs column.

now when i am entering data in CREDIT NOTE head its asking for original invoive number from which u have received this return goods.

but when i am entering that original invoice number its giving a message PROCESSED WITH ERROR .what should i do in this  case plz help..

3 such cases are in pending status.