Require the previous lesson in a LifterLMS course to be complete before moving forward. Same as requiring the prerequisite for a lesson to be the previous lesson.

require-prev-lesson-complete.php

/*
Plugin Name: LifterLMS Mods
Plugin URI: https://academy.groundhogg.io
Description: Modify default LifterLMS behaviour.
Version: 1.0
Author: Groundhogg Inc.
Author URI: http://groundhogg.io
License: A "Slug" license name e.g. GPL2
*/

function lifterlms_require_previous_lesson_be_complete( $results, $post_id ){

    if ( 'lesson' !== get_post_type( $post_id ) ){
        return $results;
    }

    if ( $results[ 'is_restricted' ] === true ){
        return $results;
    }

    $user_id = get_current_user_id();

    $student = false;

    if ( $user_id ) {
        $student = new LLMS_Student( $user_id );
    }

    $lesson = new LLMS_Lesson( $post_id );

    $previous_lesson_id = $lesson->get_previous_lesson();

    if ( $previous_lesson_id && $student ){

        if ( ! $student->is_complete( $previous_lesson_id, 'lesson' ) ){
            $results['is_restricted']  = true;
            $results['reason']         = sprintf( '%s_prerequisite', 'lesson' );
            $results['restriction_id'] = $previous_lesson_id;
        }
    }

    return $results;

}

add_filter( 'llms_page_restricted', 'lifterlms_require_previous_lesson_be_complete', 99, 2 );