A bill is not available to allocate a credit note or receipt against

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.
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If you are still unable to allocate the bill when posting the credit note and cannot see any obvious cause, please raise a new case online and reference the title of this article.