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. 

You might need to add 

skip_before_action :authenticate_user!

in the controller that is causing the error message. It may be in the user_token_controller.rb where you would need to add this.