Simplenews newsletter unsubscribe page- Access denied
The access arguments for that page are subscribe to newsletters, meaning that you need to grant that permission to anonymous users, in order to allow them to unsubscribe without logging in.

Here's how to do that:


Navigate to /admin/people/permissions.
Find "subscribe to newsletters" item.
Grant the permission to anonymous users.


Note: I just told you how to allow to do it for everyone. You need to evaluate if this fits your website and workflows.