"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.
just to know : how long did it take for you to learn so many langages ?
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!
Człowiek and człowiekiem feels absolutely impossible to pronounce. It hurts my brain and my mouth...
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".