In accordance with French legislation, it is not possible to delete an invoice in Sylob.
Invoice and credit note numbers must be :
Continuous and unbroken: no missing numbers,
Unique : no duplicates,
Chronological : an invoice 100001 must have been created before an invoice 100002.
You can create a credit note for the entire invoice by ticking the "Re-invoicing" option (in step 4 of the wizard), so that you can re-invoice the order or delivery note.