Well, firstly, if something is happening "now", you should use Present Continous, not Present Simple.
And secondly, even if there wasn't "now" at the beginning, this is one of the rare cases when Polish distinguishes between Present Simple and Present Continous. So:
to carry = nosić (Noszę kota)
to be carrying = nieść (Niosę kota)
It is perfectly fine to begin a clause in the present simple tense with the word "now". For example, "I used to carry my bag to school; now I carry a cat."
As usual, the difference is that you habitually carry a cat, but not necessarily at the current moment. The word "now" indicates you didn't do this habitually before.
None taken, but I suppose other students follow my example rather in terms of Polish, and not English ;) I'll try to be more careful. But I have this little observation, that the more I write in English, the more small mistakes I make... because I feel confident, I don't think that much about what I write, and then I end up mixing 'than' and 'then' or similar ;)
"I carry a cay now" or "now I carry a cat" should be accepted as answers as they are correct. See the following two examples: "I just carried a dog, now I carry a cat." "Back when my parents had a parrot, I carried that to the vet for the monthly check. I carry a cat now, as they have had one for a couple of years." Both are examples of sentences that make complete sense with "now" + present simple. There is no context whatsoever on Duolingo which is quite limiting, so at least one should be able to give answers that make complete sense in a given context :)
So I just asked one of our British contributors about your sentences, and to your first one she just answered "No.", while to the second "Okay, maybe but it's a bit of an unlikely context and 'take' is much more likely".
Not to mention that accepting "carry" here and in other sentences would mean that we have no way of teaching the difference between "niosę" (right now) and "noszę" (in general).
As a native English speaker, the translation of this sentence makes it sound like the person is carrying a cat 'now', as opposed to earlier when they were not. Is this what the Polish sentence is mean to imply (as opposed to merely carrying a cat in the present tense)? Thanks