I have exact same problem with Samsung J3. When you use the bitmoji app, one can pick moji then messages and converts mms allowing you to address/send. But when picking a bitmoji from GBoard, you immediately get the "text field doesn't support image insertion" error. Noting; bitmoji images display ok in gboard, allowing selection of a bitmoji, so GBoard/bitmoji apps interacting ok at some level. Which seems to imply a missing "convert to mms" step upon selection. This convert to mms process happens when doing the process; "bitmoji app, select moji, send to messages app"