In these two days.
I logged into App Store connect. And go to check my APP.
It's ok for me to check the "APP Store" tab.
But every time once I click on the "TestFlight" tab.
The browser would bring me to the login page of App Store connect.
Just can't setup the TestFlight.
Is there anything wrong? Or is there any alternative way to do it?
It sounds like you might need a bit more support to troubleshoot this issue. I would recommend that you reach out to us using the Contact Us tool on the Developer website: https://developer.apple.com/contact/topic/select
Select App Setup and Distribution > TestFlight and submit your issue and you can receive more in-depth troubleshooting to resolve your issue.