If you are using the TestFlight SDK 1.2 and greater you will need to use an App Token as opposed to a Team Token in order for the SDK to work. App Tokens help TestFlight to provide you with more accurate reporting and more flexibility as we grow and provide you with future services.
App Tokens are generated when you upload a build or create an app and will show up on the App's App Token & SDK page. Select an application from your app list and click App Token in your App navigation.
To learn about Apps, please see the article here
To learn how to implement the TestFlight SDK, please see the article here