Can someone explain the difference between "za" and "zbyt"?
they have the same meaning, but zbyt is slightly more formal, and more universal , some adjectives and most participles work only with zbyt.
Can one say "zbyt późno" too?
Yes, it could be. Personally I prefer „za późno”.
One of my favourite Peanuts scenes has Snoopy sitting on his kennel, typing his new book manuscript entitled "Things I learned after it was Too late".
So I wrote the answer and it was correct, but it does not accept it and I can't finish this whole part because of that
Such things seem to happen, that's a bug. Hopefully next time you try it will be fine.
it happens, but I always get such pages to work again by just waiting for about half a minute, maybe it'll work for you as well