I have been having this issue with my A71 for a while now. When pressing the chat box for certain apps such as the default Messages app or Snapchat, the keyboard fails to show up and the whole app freezes. The back button and centre button freeze up too. The only way to get out of it is to close all apps using the bottom left button (name?).
I've tried restarting the phone with no luck. Strangely it only seems to happen with those apps in particular.
Is the software on your phone up to date? Go to Settings > Software update > Download and install. Also, let's try a Soft Reset on the phone. This will end all processes and give the phone a fresh start. Press and hold the Volume Down and Power keys at the same time until the phone switches off and back on again.