When the commendations screen comes up at the end of an activity you are no longer able to open text chat, the only work around I know of is to whisper to someone on your friends list and then switch channels to fireteam. You can also open text chat before commendations come up, but that's not always possible depending on the activity. It's only a problem on console, but I'm pretty sure this bug has existed since commendations were literally 1st introduced.