Modify the pmpro_lesson post type args to allow comments.

modify_pmpro_lesson_post_type_args.php

<?php
function modify_pmpro_lesson_post_type_args($args, $post_type) {
    // Target only the 'pmpro_lesson' post type
    if ($post_type === 'pmpro_lesson') {
        // Merge 'comments' (discussion) support into the 'supports' array
        if (isset($args['supports']) && is_array($args['supports'])) {
            $args['supports'][] = 'comments'; // Add 'comments' support
        } else {
            $args['supports'] = array('comments'); // Create the supports array if it doesn't exist
        }
    }
    return $args;
}
add_filter('register_post_type_args', 'modify_pmpro_lesson_post_type_args', 10, 2);