You created invoices that still had an open amount. Meaning, the guest did not pay upon check-out, but will pay the invoice later. You want to track in apaleo, which of those have been paid.
Find the invoice
To find the invoice, navigate to the invoice list, and search by reservation or invoice number, guest, or company. Check "Show only checked out by AR" and "Not paid". Optionally filter by the outstanding amount or date.
Mark as paid
Find the invoice in the list, and select Mark as paid from the menu. Enter a receipt, for example a bank transfer number.