"The soap is on the bathtub."
Translation:Il sapone è sulla vasca da bagno.
Credo ch è possibile di scrivere oppure dire "bagno" (for bath/-tub) ma duolingo vuole evidenziare "la vasca da bagno"
In German you can also say that if you want to clearly say bathTUB you say "Badewanne" but just bath can mean the tub or the room itself but if you talk to friends and don't wanna be formal you say "Bad" but maybe it is because if you want to tell someone where the soap is and you say on the bath it sounds confusing or a bit stupid (in German also) because you could mean on the roof of the bathroom....
Hope you understand haha
Il sapone sta sulla vasca da bagno is also correct and more commonly used instead of the verb essere which is used to indicate the state of being of living things as opposed to the location if a bar of soap. Duo should emphasize the verb STARE in these cases. In English there is no such distinction.
Come sta tuo padre vs. Com'è tuo padre
Lui è morto NOT Lui sta morto.