In-App Purchases what am I missing? [swift]

I'm trying to learn how to implement in-app purchases. I found this really useful guide LINK which is related to this GitHub project LINK. The project works by itself I only needed to create a couple of things in order to let it work:

1. I created a sandbox account for testing (without paying :])


2. I created a new App in iTunes Connect using my bundle identifier


3. I created the in-app purchase as requested by the app


4. I created all the App IDs and Provisioning profile for the App and connected to it


5. Running the app everything goes smoothly but the only thing (and most important one) it is that the transaction fails.

I have no idea where to look since I think I've everything correctly. Someone can help me through the process reaching the end? Thanks