500 internal server error

When setting it up and creating credit bundles whenever i hit save i get the 500 internal server error. Any ideas? When i reached hout to my hosting provider they said the error in question seems to be the following: PHP Fatal error: Uncaught Error: Call to a member function set_product_visibility() on boolean in /roffs20190404/wp-content/plugins/woo-download-credits/woo-download-credits.php:969