The Trac process is unable to create a new folder due to lack of filesystem permissions. You should make sure your projects are all owned by the same user the web server runs as (often "www-data" or "apache" or "nobody" depending on the server and your distribution)
E.g:
$ chown -R www-data /var/trac
Also, check if SElinux is enabled. If so you'll need to ensure that it is configure to allow the web server process to write to the Trac projects.
If you need more help, please use the MailingList for support.
