A Pleo expense is missing a category that reflects an active expense account in Quickbooks. It's not possible to export expenses without a relevant expense account, so the export fails. It might be that you've included a category, but it's not active in Quickbooks. If a category is missing entirely, you might get the message "detail must not be null", in which case the below solution also applies.
Make sure that you've synced your Pleo categories with Quickbooks.
Add a category to the affected Pleo expense.