I signed in under xxx@ xxx.com Apple ID In the meantime, I'm connected to  App Store> Sandbox ID is yyy@ yyy.com (which was registered via "Users and Access" > "Sandbox Testers" tab) I go to the TestFlight app and pay When I confirm payment, there is a modal ask to enter a password from xxx@ xxx.com ( But it must be yyy@ yyy.com ) I go back to  App Store> Sandbox ID and see that yyy@ yyy.com is now changed to xxx@ xxx.com I click manage, and it says Cannot Connect.

There is a workaround: Install the app being logged in under real Apple id Create a sandbox email Sign out from the real Apple ID Sign in using Sandbox email instead of real apple id Open the app ( you don't need to use TestFLight!). Just open as an app from the screen. Pay from the app Open Settings - App Store - Sandbox - Manage - see your subscriptions (NOTE! to update the app to the newest version you'd need to log in back with real apple id, upload new version, repeat steps 3-7)