HitPay.Config.api_key

You're seeing just the function api_key, go back to HitPay.Config module for more information.

In config.exs, use a string, a function or a tuple:

config :hit_pay, api_key: System.get_env("HIT_PAY_API_KEY")

or:

config :hit_pay, api_key: {:system, "HIT_PAY_API_KEY"}

or:

config :hit_pay, api_key: {MyApp.Config, :hit_pay_api_key, []}