Not really since it isn't being used elsewhere in the class. Keep in mind that the memorization is only valid for the lifecycle of the request and any further requests would no longer have the calculations stored. 

It should be user_signed_in?, not user_signed_in.

What error are you receiving? 

If I remember correctly, the authenticate_user has some weird interactions with JWT. This was part of the reason for the alias methods in the User model. Part of the solution was to undefine the method current_user and and with the includes of the Knock Authenticatable, the current user would be set.

Are you receiving any specific error messages?

That does appear to be a devise error message. If I had to guess, your controller/action that is redirecting/responding you to that error message is not inheriting from the Api::ApplicationController.