Get LearnDash next lesson link or first lesson link if course not started

functions.php

function ld_next_lesson_link( $course_id = null ) {
	global $post;
	$user = _wp_get_current_user();

	if( is_null( $course_id ) ) {
		$course_id = learndash_get_course_id( $post );
	}

	if( !$course_id || !isset( $user->ID ) )  {
		// User Not Logged In OR No Course Identified
		return false;
	}

	$lessons = learndash_get_lesson_list( $course_id );
		
	if( !$lessons ) {
		// No Lesson
		return false;
	}

	$first_lesson = reset( $lessons );

	$user_course_progress = get_user_meta( $user->ID, '_sfwd-course_progress', true );

	if( isset( $user_course_progress[$course_id] ) ) {
		$course_progress = $user_course_progress[$course_id];

		// get first lesson link
		if( !$course_progress['lessons'] && isset( $first_lesson->ID ) ) {
			$lesson_id = $first_lesson->ID;
		} else {
			end( $course_progress['lessons'] );
			$lesson_id = key( $course_progress['lessons'] );
			
			foreach( $lessons as $key => $lesson ) {
				if( $lesson->ID == $lesson_id ) {
					$lesson_id = $lessons[$key+1]->ID;
					break;
				}
			}
		}

	} elseif( isset ( $first_lesson->ID ) ) {
		// get first lesson link
		$lesson_id = $first_lesson->ID;
	}

	if( !$lesson_id ) {
		// No Lesson ID
		return false;
	}

	if( 'sfwd-lessons' != get_post_type( $lesson_id ) ) {
		// ID not for a Learndash Lesson
		return false;
	}

	$link = get_post_permalink( $lesson_id );
	return $link;
}

function shortcode_ld_next_lesson_link( $atts , $content = 'Next Lesson' ){
	extract(shortcode_atts(array(
		'course_id' => null ,
		'class' => 'learndash-next-lesson'
	), $atts));
	$url = ld_next_lesson_link( $course_id );
	if( $url ) {
		return '<a href="'.$url.'" class="'.$class.'">'.$content.'</a>';
	}
	return false;
}
add_shortcode('ld_next_lesson_link', 'shortcode_ld_next_lesson_link');

Thank you very much! It solves perfectly my problem!
:)

Thank you, this was very helpful. I added an additional function to grab the first topic id from the lesson id returned in your function.

`function ld_first_topic_id( $lesson_id = null, $course_id = null ) {

	if( empty($lesson_id) || empty($course_id) ) {
		return false;
	}

	$topics = learndash_get_topic_list( $lesson_id, $course_id );

	if( empty($topics) ) {
		return false;
	}

	$first_topic = reset( $topics );

	if( empty($first_topic->ID) || 'sfwd-topic' != get_post_type( $first_topic->ID ) ) {
		return false;
	}

	return $first_topic->ID;

}`