PHP Memory Limit
The PHP Memory Limit is the amount of memory a script is allowed to allocate, per the PHP Documentation on this setting. Suggestions on setting this value are provided in PHP memory limit and Zen Cart.
In some environments, changing your PHP version can reset your memory limit setting. Go to the version page after updating PHP to verify the setting of your PHP Memory Limit.
The process of updating your PHP memory is server-specific, depending on whether your hoster uses PHP Selector, MultiPHP Manager or some other mechanism. See PHP Configuration for more information on setting PHP configuration.
Your hoster likely provides a knowledge base with instructions. Contact your hoster if you are unsure how to proceed.