JaredGriz is right. You can't do that. But if your bilingual in two languages that aren't of your own invention, apply here! https://incubator.duolingo.com/apply

I'm not sure if you can create your own language and put it on here. But I think you can request to add languages if you contact someone like a moderator.

For making a course for your own language, try Memrise or TinyCards or Anki. These are all flashcard apps. I believe full sentences can be put into them all, so a course something similar in idea to Duolingo's courses can be built. If you haven't looked at how to create a language before, then zompist's Language Construction Kit is a good resource.

Duolingo really isn't set up for everyone to start making their own language.

If you are interested in making your own language on Duolingo, I'd recommend TinyCards. It's run by Duolingo and you don't need permission to create it.