"I am a human."
I believe it's not wrong from the grammatical standpoint but it sounds really unnatural. This to + nominative construction is usually used with the third person (singular as well as plural). It can also sound unnatural in shorter sentences e.g.: "On jest inżynierem (much better) / On to inżynier".
But you can say things like: "Pies to niebezpieczne zwierzę" or "Ta książka to arcydzieło". Note that jest + instrumental construction also works here just fine.
In fact I've only learned French and German (outside Duolingo with other resources) for about 1.5 years and I'm still learning them. I'm also planning to start learning Russian. I love learning languages so I finish these trees, too. I've been on Duolingo for less than 2 years. After finishing the tree I can usually honestly say I know this language at the basic level. My goal is just to get a grasp of this language and to get familiar with it. If I particularly like some language I've done I can always learn it in future, already having a strong base for it. But I hope (like all of us) I'll learn most of them fluently someday :) Powodzenia z polskim!
I guess it was just too many typos (well, 2) for the system.
Firstly, this second C isn't there, and secondly, the letter ł (Ł) isn't a variant of T. It's a variant of L. So if you don't have Polish letters (but it's really recommended if you use them), at least use L (czlowiek). So the full word here is "człowiekiem".