The error "Invoice not of valid status for modification" is returned by Xero when Bookeo tries to sync an update to an invoice.
The most common cause for this error message is that the invoice was voided in Xero. It is recommended to avoid voiding invoices in Xero for invoices created by Bookeo. Instead, you should cancel the booking in Bookeo, which will also void the invoice in Xero.
If the Xero invoice is not of "VOID" status, the error message may be caused by something else. In this case, please contact Xero support to inquire about the exact reason for the error message.