Hello HRH,
Thank you for your message, it's something that would normally only happen if there was reason for the link to not occur, such as incorrect supplier or the credit was already linked against something else.
Does the Supplier Credit have precisely the same supplier in as the Bill in question?
Alternatively, try using a Cheque to link the supplier Credit to the Bill, you may have better success with that - when you select the bill and credit it will convert itself automatically to a "pay bills" transaction.