During installation of any of our products, you may see this message "config.php" is not writable in the first step. It can happen because of 2 reasons, I'll explain them and show you shot to solve the issue -
1. It can happen because wrong permission for the "config.php" file, the permission should be 644. So if you face the issue check if "config.php" file has 644 as permission or not, if you see it's not 644 then change it to 644.
2. It can happen because of file/folder owner mismatch like if you are running an Apache server and your web root directory (/var/www/html) is owned by root that may create problems too. For that please change the owner of web root directory (/var/www/html) to Apache from root or anything else, you can do this from FTP client or SSH. After that please restart Apache server and you should be fine.
in FTP client it should look like similar to this
Change the owner to apache and then restart Apache server.
run this command: chown -R apache:apache /var/www (you need to replace /var/www if your web root directory is different)
then restart Apache web server, to do that run this command: service httpd restart
If you still have issues or questions feel free to create a support ticket here.