try,

CREATE
DEFINER=`root`@`localhost`
TRIGGER `meemor`.`addnotification`
AFTER UPDATE ON `meemor`.`posts`
FOR EACH ROW
BEGIN
  DECLARE _user_id BIGINT;
  DECLARE no_more_rows BOOLEAN;
  DECLARE loop_cntr INT DEFAULT 0;
  DECLARE num_rows INT DEFAULT 0;
  DECLARE new_post INT;
  set new_post = NEW.post_id;
  DECLARE notifition_group CURSOR FOR select user_id FROM posts WHERE post_id = new_post;
  DECLARE CONTINUE HANDLER FOR NOT FOUND SET no_more_rows = TRUE;
  
  OPEN notifition_group;
  
  the_loop: LOOP

    FETCH notifition_group
    INTO _user_id;
    
    IF no_more_rows THEN CLOSE notifition_group; LEAVE the_loop; END IF;

    
    SET loop_cntr = loop_cntr + 1;

    INSERT INTO notifications (user_id)
        VALUES(_user_id);

  END LOOP the_loop;
  END$$