The reason for this error is that your server configuration has a very low memory limit. You need to increase PHP Memory Limit in WordPress.

First, edit the wp-config.php file on your WordPress site. It is located in your WordPress site’s root folder, and you will need to use an FTP client or file manager in your web hosting control panel.
Next, you need to paste this code in wp-config.php file just before the line that says ‘That’s all, stop editing! Happy blogging.’


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


This code tells WordPress to increase the PHP memory limit to 256MB.
Once you are done, you need to save your changes and upload your wp-config.php file back to your server.
You can now visit your WordPress site and memory exhausted error should disappear now.

Note: If this solution does not work for you, then this means your web hosting service provider does not allow WordPress to increase PHP memory limit. You will need to ask your web hosting provider to increase your PHP memory limit manually.


Source: http://www.wpbeginner.com/wp-tutorials/fix-wordpress-memory-exhausted-error-increase-php-memory/