Why can't I say "me emme"?
You can, Duolingo just doesn't know that yet. Remember to report these so that the mods can add them as alternative answers eventually :)
I had the same problem.
I also reported the problem of "me emme"...
Me too. Hopefully it gets added soon!
Can I say että instead of vaan?
No. Että is one of the translations of the English that.