In Gothenburg you need to have a valid ticket while travelling by tram or bus within the city. You can only pay for your ticket by debit/credit card or by purchasing your ticket in the Västtrafik To Go App before you enter the vehicle. No cash can be used.
You will find more and useful information on the Vasttrafik (Västtrafik) website:
https://www.vasttrafik.se/en/Tickets/more-about-tickets/kopa-ticket-on-board/ >