The error "Payment not of valid status for modification" is returned by Xero when Bookeo tries to sync an update to an invoice.
If a Xero invoice already has payments, Xero does not allow changing the invoice. In these cases, Xero requires to delete all payments from the invoice, changing the invoice, and then adding the payments again. This is what Bookeo does when it needs to update an invoice in Xero because of changes in the booking.
If you get this error message however, it means that Xero did not allow deleting a payment for the invoice. This can occur for example if the payment was already reconciled, but there could be other reasons, which only Xero can provide.
Since it is not possible to change an invoice with payments, and it is not possible to delete this payment from the invoice, this error message that Xero will not allow changes to this invoice.
For more help on how Xero limits changes to invoices and payments, please contact Xero support directly.