Your Eventsforce event must be "Live" in order for public attendees to visit your event website and register. If your event is "Not live" when providing the registration link, attendees will see an error saying "Sorry, your page request could not be completed" (example below):
Making your event live
Go to Setup > Event > Properties
Select "Live" as the 'Status'
Once the event is live, you will see a green "Live" banner in the top left corner of your screen: