Temporary fix I found for people who need to see their apps on the test devices. I found that you can go into testflight->click on the problematic app-> click on previous builds. From there you can load a previous build and update that build with the update button or simply click install on the most previous build. This allowed me to download and test the most recent build for a few sessions until the message returns again. If it returns you can just redo those steps and it should download again