This policy should be bound to the stage after your redirect should happen. For example, if you have an identification and a password stage, and you want to redirect after identification, bind the policy to the password stage. Make sure the policy binding is set to re-evaluate policies.
When used with authentik 2022.7 or later, set the flow _Denied action_ to _CONTINUE_. This will redirect already authenticated users to the default interface if they try to use the respective flow.