Fitting a whole water melon into your mouth is impossible. Trying to make yourself invisible is impossible. A person, however, cannot be impossible.
But it’s a mistake that some students make when they are having a bad English lesson. When they can’t think of the right vocabulary, or they make silly mistakes, they say, „I am impossible today.“ But this is a very incorrect sentence.
What they mean is that they are ‚hopeless‘ or useless‘, or you can even say ‚terrible‘. You cannot say impossible. Tasks can be impossible, but a person, especially one learning English, cannot be!