Event Info Page error with Joomla
This is an ACL issue. If users can login, then register it is simpler. 
If you are allowing anonymous users then you have a tiny bit of configuration to do to enable anonymous users.
Have to race out to a client for the day, i can send you info tonight with some screen captures, but also maybe you can look up the instructions in the manuals, it works quite nicely when configured.
CiviCRM » Administer CiviCRM » Access Control

For more reference please give a read https://book.civicrm.org/user/current/initial-set-up/permissions-and-access-control/

Also can you just double check that you have the event checked off as a "public event".