After logging in and going to checkout or just going to check while being logged in, I'm getting this fatal error


Fatal error: Call to undefined function: zen_get_customer_validate_session() in /Volumes/Files/MCzenstore/includes/modules/pages/checkout_shipping/header_php.php on line 27


I've search and replaced to try to fix this and continue, but I can't find the error.

Any help would be appreciated