Note: Recently, some users reported that testflight will report an error when opening. Here is a solution: open the iPhone, click on the desktop-select settings-click wifi-click the small exclamation mark on the right side of the current connection wifi, drop down to select configure DNS, click select manual. Delete other items in the DNS server below, add Google DNS of 8.8.4.4, save it and open Testflight again to use it.