Complet shutdown website by this plugin

[28-Nov-2019 10:34:37 UTC] PHP Warning: call_user_func_array() expects parameter 1 to be a valid callback, class ‘YITH_WC_Points_Rewards_Frontend’ does not have a method ‘print_rewards_message’ in /home/natuurl6/public_html/wp/wp-includes/class-wp-hook.php on line 288

Please help me why does this happen with the free version?
What do I need to do or not to do?

https://www.natuurlijk4dogs.nl

The page I need help with: [log in to see the link]