update_post_meta in admin folder

I’m trying to update a custom_post field in admin aera,

I have a sigle page validate.php with this code,

$post_id=$_GET[“idreserva”];
update_post_meta($post_id,'wpcf-status','1');

If trye with in this way
update_post_meta($post_id,'wpcf-status','1');

But it dosen’t work, Is the first time I work inside admin folder, may be is not a good way?

any idea why update_post_meta is not working?

Thanks, Best regards