Hello,
Please make sure all the url and variable are in small caps. also try to append redirect_to variable with the url you wish your users to redirect on after logout so your complete url will look like the following.
if you are running WordPress in the root of your domain:
yourdomain.com/?piereg_logout_url=true&redirect=http%3A%2F%2Fyourdomain.com%2Fthank-you
if you are running inside a subfolder:
yourdomain.com/wordpress-folder/?piereg_logout_url=true&redirect=http%3A%2F%2Fyourdomain.com%2Fwordpress-folder%2Fthank-you
Thanks