What to do if you see the message "When you use Creditors, you must choose a supplier."

The cause:

When a Pleo invoice is exported, the listed supplier is listed as a debtor instead of a creditor in Quickbooks. This causes the export to fail, as a Pleo expense doesn't contain a customer field (which is mandatory for a supplier listed as a debtor).

The solution:

In Quickbooks, edit the supplier so that they're listed as a creditor instead of a debtor.