I can’t post comment via REST API

I do not recommend doing this. It’s better if you authenticate your users, but you can make an api endpoint that posts a comment like this:

add_action('rest_api_init',
    function () {
    register_rest_route(
        'api',
        'api_post_comment',
        array(
            'methods'  => 'GET',
            'callback' => 'api_post_comment',
        )
    );
});

function api_post_comment() {
    wp_insert_comment([
        'comment_approved' => 0,
        'comment_author' => 'test',
        'comment_author_email' => '[email protected]',
        'comment_author_IP' => $_SERVER['REMOTE_ADDR'],
        'comment_content' => 'test',
        'comment_parent' => 0,
        'comment_post_ID' => 25084,
    ]);
}

You can learn more here:
https://developer.wordpress.org/reference/functions/wp_insert_comment/

and here:
https://developer.wordpress.org/rest-api/extending-the-rest-api/adding-custom-endpoints/