Permission denied.
Your webserver is not authorized to access config/groups.php.
It appears that you're trying to run FluxCP from your home directory (a very bad idea). When you install apache it creates /var/www for you and makes that the default serve from directory. Running a webserver out of your home directory means anything in your home directory (including configuration files) are up for grabs.
To fix it from your current configuration, you would have to add yourself to the www-data group, then chgrp all the files in your home directory to www-data.
For the CORRECT fix, move all your flux CP files to /var/www, reset the appropriate apache configuration to serve files from there as it is supposed to be, then add your username to the www-data group to be able to modify files in the /var/www directory.
