I've been having this same problem for a week or so now. I was able to get the save button to be enabled by taking the following steps:
Before you upload your build:
Log in iTunes Connect, select My Apps, select "[your app]", select TestFlight, then under Internal Testin (on the left), select "Not Available for Testing."
Upload your build, then wait for "processing" to disappear next to your build number and for the build number to turn blue and clickable under Activity.
Then go to TestFlight > Internal Testing and select the new build number which should be there and clickable above "Not Available for Testing." Then try to edit the info. The save button turned blue for me but when I clicked it did nothing.
Who uploaded the build originally? I wonder if that has anything to do with it. I uploaded the version as an App Manager then I changed my role to Admin and its been giving me this problem near the time of th role switch. Wonder if that has anything to do with it. Good Luck!