@YoungMaster Welcome to Microsoft Q & A Community Forum. As you mentioned, you are trying to create a custom policy and I assume you are using "DeployIfNotExists" effect. You are prompted with above error as the api version of the resource type 'namespaces/authorizationrules' which you have provided is incorrect. The error message already contains the list of all supported versions. Update it to correct version and retry the custom policy.
To check the list of api versions available for 'namespaces/authorizationrules', refer this document.