Snippet to automatically mark LearnDash Lessons and Topics as complete when the user clicks "Next Lesson" or "Next Topic," eliminating the need to manually click "Mark Complete."

functions.php

/**
 * Handle the AJAX request to mark a LearnDash topic or lesson as complete.
 *
 * @return void
 */
function custom_evolve_handle_ld_mark_topic_complete() {
    // Verify the nonce for security.
    check_ajax_referer( 'ld_complete_nonce', 'nonce' );

    // Get and sanitize input data.
    $post_id = isset( $_GET['post_id'] ) ? absint( $_GET['post_id'] ) : 0;
    $user_id = isset( $_GET['user_id'] ) ? absint( $_GET['user_id'] ) : 0;

    // Validate the post ID and user ID.
    if ( ! $post_id || ! $user_id ) {
        wp_send_json_error(
            array(
                'message' => esc_html__( 'Invalid post or user ID.', 'text-domain' ),
            )
        );
    }

    // Mark the topic or lesson as complete for the user.
    learndash_process_mark_complete( $user_id, $post_id );

    wp_send_json_success(
        array(
            'message' => esc_html__( 'Marked as complete.', 'text-domain' ),
        )
    );
}
add_action( 'wp_ajax_mark_ld_lesson_topic_complete', 'custom_evolve_handle_ld_mark_topic_complete' );

/**
 * Enqueue inline JavaScript for handling LearnDash "Next" button actions.
 *
 * @return void
 */
function custom_evolve_enqueue_footer_script() {
    // Check if the current post type is a LearnDash post type.
    $learndash_post_types = array( 'sfwd-lessons', 'sfwd-topic', 'sfwd-quiz' );
    if ( ! in_array( get_post_type(), $learndash_post_types, true ) ) {
        return; // Exit early if not a LearnDash post type.
    }

    // Prepare data for the inline script.
    $script_data = array(
        'ajax_url' => esc_url( admin_url( 'admin-ajax.php' ) ),
        'nonce'    => esc_js( wp_create_nonce( 'ld_complete_nonce' ) ),
        'post_id'  => esc_js( absint( get_the_ID() ) ),
        'user_id'  => esc_js( absint( get_current_user_id() ) ),
    );

    // Convert data to a JavaScript object.
    $script_data_json = wp_json_encode( $script_data );

    // Add inline script.
    $script = "
        jQuery(document).ready(function($) {
            var data = {$script_data_json};

            // Target buttons with specific span text (Next Quiz, Next Topic, Next Lesson).
            $('span.ld-text:contains(\"Next Quiz\"), span.ld-text:contains(\"Next Topic\"), span.ld-text:contains(\"Next Lesson\")')
                .closest('a.ld-button')
                .on('click', function(e) {
                    // AJAX request to mark the current topic or lesson as complete.
                    $.ajax({
                        url: data.ajax_url,
                        type: 'GET',
                        async: false,
                        data: {
                            action: 'mark_ld_lesson_topic_complete',
                            nonce: data.nonce,
                            post_id: data.post_id,
                            user_id: data.user_id,
                        },
                        success: function(response) {
                            if (!response.success) {
                                console.error(response.data.message);
                            }
                        },
                        error: function(xhr, status, error) {
                            console.error('AJAX error:', error);
                        },
                    });
                });
        });
    ";

    // Enqueue the inline script.
    wp_add_inline_script( 'jquery', $script );
}
add_action( 'wp_enqueue_scripts', 'custom_evolve_enqueue_footer_script' );

Simply add this code in your child theme's functions.php file and the functionality will start to work