I always helps to post the full and complete error message, as text if you can copy/paste it, but a screen shot if you can't.
You mentioned certificate; do you have Subversion running under HTTPS?
Also, how are you attempting access? Via the svn client app, through and
IDE, or through TortoiseSVN?
Usually, the first time an Subversion client connects to a Subversion repository, and that repository requires credentials, as soon as you sign into the repository your credentials are stored on your mahcine and then reused.
You mentioned Subversion if running on CentOS, but what are you running on?