Before you use your membership for the first time be sure that you have logged into the app successfully using the password and email address you signed up with over the home page. Before you can check in you need to upload a profile picture, so you can be identified at the partner venue, either in the app or from your profile on the website. Check you have a mobile internet connection and that you have given the app access to your camera.
Once you've done that then you should have no problem checking in at the partner location.