Why doesn't my Credit Note display the line items contained on an invoice?

In order for a Credit Note to display these line items, the Invoice line items must be linked to the line items on the Purchase Order as seen below:
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1. To link a line item, ensure that your Invoice is matched to a Purchase Order.

Once it has been linked, double-click the 'unlinked' box associated with that line item and select the corresponding line item from the Purchase Order:

2. Once your line items are linked, select '+ Credit Note' in the bottom right of the Invoice screen and follow the steps outlined in Creating a Credit Note.
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