If a credit note was posted against an original invoice, it's purpose is to reduce the amount owing for the invoice, and you cannot select it for payments, nor print it on the advice.
If you want to be able to select a credit note and see it on the payment advice, it must be entered without the original document number, instead leaving it blank. Once the credit note has been posted, it is too late to change this (apart from posting a debit note against the invoice to bring it back to the original amount, then posting another credit note not applied to anything).
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Regards,
Softrak Tech Support