Language /
Nominative (Mianownik) vs Accusative (Biernik)? [17]
Why is kąpiel wrong?
Because it's "taking a bath", the name of the activity.
Siedzę w kąpieli - I'm sitting in a bathtub full of water.
Kąpiel gotowa - the bath is ready.
"Kąpiel" basically means getting wet.
The thing that you have in your bathroom is called wanna. Kąpiel w wannie - taking a bath in a bathtub. But kąpiel w jeziorze means swimming in a lake.
Why can't I just copy (the nominative) from the dictionary?!
You can, but the dictionary doesn't know which meaning of a word you need. You looked up "bath" and got "kąpiel". If you had checked "bathtub", you'd have got "wanna". Best to get a dictionary which gives examples of usage in sentences, this helps a lot.
I'm sure you'll do better next year :)