I'm having the same problem. I spoke with a friend of mine. He's an iOS developer with 10+. He said he thinks it's a general error, as if there's a system problem, and he can't install it either. I think there's a problem with Apple's systems, and they definitely haven't found a solution. I'm curious to see how this problem will be resolved.
Topic:
App Store Distribution & Marketing
SubTopic:
TestFlight