How can external users test our watchOS 2 beta apps throught the updated TestFlight app if this message is showing up?
External testing is not supported for builds that have been created with a beta version of Xcode, OS X, or iOS.
They can't (yet).
Apple has sent developers an email which clarifies that the TestFlight 1.2.0 update does not yet extend to support external test groups.
External user testing would require that the app go through Beta App Review - whereas this is not necessary for internal user testing. Evidently Apple is not ready to provide Beta App Review for iOS 9 and watchOS 2 apps yet.
Max.
UPDATE: That changed today - Apple are now accepting submissions for Beta App Review so you should be able to push your apps out to external testers soon 🙂
Update: they have switched it back off.
It looks like that the fact that external testing was available for a short period of time is nothing more than just another iTunes Connect bug 😟
Are you serious
Yep, smh. I can't even upload a build to iTunes Connect now without getting a ERROR ITMS - 90125 : "Binary Invalid"
Same error here, ERROR ITMS-90125 when trying to submit internal beta that includes watch OS 2 app