Thank you for your request.
It is only possible to book a ticket online for 5 people at once.
If you had difficulties booking the last and sixth ticket, I recommend you to close the page, delete browser data and open the page again on another browser. If this does not work please call us: Tel. 0848 44 66 88,(CHF 0.08/min.),(available 24/7, sometimes you may be required to wait) or if your calling from abroad: +41 51 225 78 00.Or contact us via contact form: Claims and complaints | SBB help & contact.
Have a nice day
Kind regards MelanieD