Why is " a tree is no machine" wrong?
A tree's not a machine isn't working either.
It indeed tests incorrect. I don't know why, I can only say that Duolingo treats contractions automatically.
a tree is not a machine
Failed for me :(
In which way did it fail? Make a report. If it is not possible, show us a screenshot.
I clicked the "Report" button as well.
Now I can see what happened. In that exercise you were asked to type the Czech sentence you hear, not to translate it to English.