ActionController InvalidAuthenticityToken in Api::V1::UsersController#create

You need to make the following change in application_controller.rb

Change

class ApplicationController < ActionController::Base
  # Prevent CSRF attacks by raising an exception.
  # For APIs, you may want to use :null_session instead.
  protect_from_forgery with: :exception
end

to

class ApplicationController < ActionController::Base
  # Prevent CSRF attacks by raising an exception.
  # For APIs, you may want to use :null_session instead.
  protect_from_forgery with: :null_session
end

EDIT

Better way is to skip the authentication for a specific controller

class Api::V1::UsersController < ApplicationController
  skip_before_action :verify_authenticity_token

  respond_to :json
  # ...
end