Check out Anaplan's official postman collection here.
When using CA certs you are required to pass the public cert in the Authorization header like you have done, but it also requires a body that contains an encoded string of random data as well as that string signed with the associated private key to the public cert that has been registered on the Anaplan tenant. How to sign the string is laid out in our apiary.
Basic authentication involves sending a verified username and password with your request. In the request Authorization tab, select Basic Auth from the Type dropdown list. Enter your API username and password in the Username and Password fields. For additional security, store these in variables.