The issue is a negative payment amount which your version of QB will not support. To find it,
- Go to Billing Admin > Invoices & Payments > Payment Sum tab
- Generate a report for the month of the IIF file you tried importing to QB
- Look in the refund column for a negative amount
So QB does not allow this amount to be negative; i.e., a negative payment. It's fine in SwimOffice but not QB. Did you write a refund check or was it a CC refund?
Did any of the IIF file go into your QB? If not TeamUnify can manually adjust the IIF file so it will import and you can manually enter in your refund amount on your QB side. Just email firstname.lastname@example.org with the details.