If a bill is not available for allocation when posting a credit note, then check that the relevant bill has not been receipted or written-off.
Reverse the relevant receipt and move funds if needed. The bill should now be available for allocation against the credit note.
If you are still unable to allocate the bill when posting the credit note and cannot see any obvious cause, please ask a question in the MyAccess Portal Digital Assistant and ask to "Speak to a human" to be put through directly to our support team.