In most cases, this happens because the PHP memory limit configurations are less than recommended. You can see your current configurations from Tools > Site Health > Info > Server 

Increasing your server PHP memory limit will solve the problem. If you don’t have an idea about it. Please contact your hosting provider and they will help you to increase it. But if you are familiar with it then increase the memory limit from the wp-config.php file.

define('WP_MEMORY_LIMIT', '512M');

You can check this video tutorial below for more clarification –

But after following the above solution if you still have the issue then contact our support team. They will guide you to solve this issue as soon as possible.