I'm sending my client a build through Test Flight (via iTunes Connect) and he's receiving this error in Test Flight app when opening link coming from iTunes Connect
This may be caused by a number of reasons.
In any way, just resend a new link. You can do that by following point 1.
I hope that helps.
Julian
Edit: As the asker himself added as comment, only adding a user won't make the trick. You have to enable Internal Testing for him under users and roles and also turn on internal testing for the app too. The problem was resolved, as the "client" was sharing his link and using it twice, even that doesn't work