CiviCRM event registration is failing
The problem is almost certainly a misconfiguration of your domain names or your SSL certificate.  Please look (carefully!) at the domain name of the page that is telling you that you don't have permission.  Then click "Back" to the page from which you clicked the link and look at the domain name there.


Is one page using SSL and one not?  E.g. does one address start with "http://" and the other with "https://"?
Does one address start with "www" and one not?  E.g. http://www.example.org and http://example.org?


If so, that's your problem.  As for fixing it - please update your question with your CiviCRM version and the CMS software/version, along with which of the two problems you're seeing, and someone should be able to point you to a fix.