The reason for this error is that your server configuration has a very low memory limit. Try adding this to wp-config.php (put it after <?php in this file):

define('WP_MEMORY_LIMIT', '96M');

Please note that this limit is OK for the theme and the plugins that come with the theme. If you want to enable other plugins, you may need to increase the limit further. Also, note that often the Hosting Company have server-wide limits that may be lower than the one set in the php.ini file. If 256MB doesn't work for you, please check with the Hosting company if this is the case. 

define('WP_MEMORY_LIMIT', '256M');

The official WordPress guide on this topic can be found here: