Bug in iTunes Connect preventing TestFlight submit
There might be a problem with iTunes Connect.  I just got an email from Apple about upcoming maintenance: 


  On April 22, beginning at 7:00 a.m. (PDT), iTunes Connect and related content delivery services will be unavailable for up to eight hours.  The following services will be affected:
   - App build upload in Xcode
   - Application Loader and altool
   - iTunes Connect
   - iTunes Connect Mobile
   - iTunes Producer
   - Transporter


Maybe wait until after the maintenance is over and try again.