Error: Bill cannot be exported as it already exists in Xero in a non-modifiable status
This message does not mean that you've already exported the very same invoice/bill. Rather, it means that an invoice with the same number already exists in Xero. It's not possible to have more than one invoice with the same number live in Xero, and so the export from Pleo fails.
Click "view expense" to see which invoice has failed to export. Next, search for the same invoice number in Xero. Once you've found it, please change the invoice number (of the one that's already in Xero) to something else. Then try exporting again.