Replace REST API with @wpgraphql with @apollographql client.

learndash-graphql.php

<?php
add_filter( 'register_post_type_args', function( $args, $post_type ) {

if ( 'sfwd-courses' === $post_type ) {
	$args['show_in_graphql'] = true;
	$args['graphql_single_name'] = 'course';
	$args['graphql_plural_name'] = 'courses';
}

if ( 'sfwd-lessons' === $post_type ) {
	$args['show_in_graphql'] = true;
	$args['graphql_single_name'] = 'lesson';
	$args['graphql_plural_name'] = 'lessons';
}

if ( 'sfwd-quiz' === $post_type ) {
	$args['show_in_graphql'] = true;
	$args['graphql_single_name'] = 'quiz';
	$args['graphql_plural_name'] = 'quizs';
}
if ( 'sfwd-topic' === $post_type ) {
	$args['show_in_graphql'] = true;
	$args['graphql_single_name'] = 'topic';
	$args['graphql_plural_name'] = 'topics';
}

return $args;

}, 10, 2 );


add_filter( 'register_taxonomy_args', function( $args, $taxonomy ) {

if ( 'doc_tag' === $taxonomy ) {
	$args['show_in_graphql'] = true;
	$args['graphql_single_name'] = 'documentTag';
	$args['graphql_plural_name'] = 'documentTags';
}

return $args;

}, 10, 2 );


add_action( 'graphql_register_types', function() {
register_graphql_field( 'Courses', 'featured_media', [
	 'type' => 'String',
	 'description' => __( 'The featured_media of the course', 'learndash' ),
	 'resolve' => function( $post ) {
		 $featured_media = get_post_meta( $post->ID, 'featured_media', true );
		 return ! empty( $featured_media ) ? $featured_media : 'null';
	 }
] );
} );


add_action( 'graphql_register_types', function() {

$post_types = WPGraphQL::$allowed_post_types;

if ( ! empty( $post_types ) && is_array( $post_types ) ) {
	foreach ( $post_types as $post_type ) {
		$post_type_object = get_post_type_object( $post_type );

		if ($post_type === 'sfwd-courses') {
			register_graphql_field( $post_type_object->graphql_single_name, 'blurb', [
				'type' => 'String',
				'description' => __( 'The blurb of the course', 'sfwd-courses' ),
				'resolve' => function( $post ) {
					$blurb = get_post_meta( $post->ID, 'blurb', true );
					return ! empty( $blurb ) ? $blurb : '';
				}
			]);
		}

		if ($post_type === 'sfwd-quiz') {
			register_graphql_field( $post_type_object->graphql_single_name, 'total', [
				'type' => 'String',
				'description' => __( 'The total of the quiz', 'sfwd-quiz' ),
				'resolve' => function( $post ) {
					$price = get_post_meta( $post->ID, 'total', true );
					return ! empty( $price ) ? $price : '';
				}
			]);
		}
	}
}
});