How to add a custom function to a wordpress site theme?

I want to add a custom function to my wordpress site functions.php file like this :

//Test adding basic REST API custom endpoint.
/**
 * Grab latest post title by an author!
 *
 * @param array $data Options for the function.
 * @return string|null Post title for the latest,  * or null if none.
 */
function my_awesome_func( $data ) {
    $posts = get_posts( array(
        'author' => $data['id'],
    ) );

    if ( empty( $posts ) ) {
        return null;
    }

    return $posts[0]->post_title;
}


add_action( 'rest_api_init', function () {
    register_rest_route( 'walden/v1', '/author/(?P<id>d+)', array(
        'methods' => 'GET',
        'callback' => 'my_awesome_func',
    ) );
} );

But wherever I paste this code it causes the whole functions to fail and all rest api links give me error .

Exactly where I should paste this piece of code in functions.php so it won’t ruin all the other functions ?

Answer

I Figured out the only problem was actually the name of the function itself , so I changes the name of the function and it works perfectly now