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.

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

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

The theme and plugins require PHP version 7.0 or higher. Now is the time to think about upgrading to PHP 7.0 if you haven’t already. Not only because you want to be running on supported software, but you are guaranteed performance benefits!  If your current WordPress host doesn’t support PHP 7.0 yet, we strongly encourage you to seek out new hosting.