Did you setup value at field "Credit Memo Nos" yet ? I think you should setup value at field "Credit Memo Nos" at page "Sales & Receivables Setup (459)"
You need to fill in the "Posted Credit Memo Nos." field on "Sales & Recievables Setup".
Once you've done this you can try posting it again but I think you may still get the error in which case you'll need to delete the Credit Memo and create it again.
Answers
Once you've done this you can try posting it again but I think you may still get the error in which case you'll need to delete the Credit Memo and create it again.