Error: "The supplier does not exist in Xero"
This message means that the supplier ID cannot be found in Xero (possibly because it was manually deleted). All suppliers need to exist in Xero before respective invoices can be exported, so the export will fail in this case.
In Xero, re-add the supplier.
Go to the Export page in Pleo.
Within the Unexported queue, filter for invoices.
Once you've found your invoice, select another supplier. This action will replace the deleted supplier in Pleo's database, meaning that the export should work. Please note that the supplier should of course be the actual vendor who issued the invoice.