I have the same bug as well. I submitted the first build of my app to TestFlight (it's not been put onto the store yet) just yesterday. The next day when I had time to install and test it I saw that the build had become expired. I uploaded another build, which appears to have been accepted without issue, created an internal testing group, found it on TestFlight, and then "The requested app is not available or doesn't exist." What????

I was having similar issues which, in my case, appear to have been caused by connecting through a VPN. After disconnecting and going through again, everything worked fine.