In American English, bathroom is a euphemism for toilet. When somebody asks this question in a restaurant they are not looking for a room with a bathtub or a shower.
In my impression, the Czech language is a bit more direct and is not afraid of saying “WC” [vɛːt͡sɛː] or even “záchod.” Can anybody say how the present (Czech) sentence would/could be understood with respect to this difference?
Koupelna is a place where you can bath or at least shower. Certainly not a toilet. It can also include a toilet bowl, then it is koupelna combined with záchod/toaleta/WC. But it must contain a bathtub or a shower. You never call a place with just a toilet bowl and maybe also with a sink "koupelna".
What I intend to convey is that 'toilet' is from the French for to wash, as lavatory is from the Latin for the same. Other euphemisms used are 'bathroom' and 'washroom' again with the same meaning. It was only to the English I was referring. My knowledge of Czech is unfortunately not yet at a standard that I am able comment on the derivation of words.
I would add, for the benefit of non-native English speakers, that "bathroom" is not always a euphemism for "toilet." It is, in the usage that you mentioned, as well as one or two others. For the most part, though, "bathroom" usually means what VladaFu has described. (And, at least in the US, the room usually includes sink and a toilet as well.)
Either you made a mistake or it is the dreaded Duolingo grading bug. We can't do anything. You did not even press "My answer should be accepted". You can report it at https://support.duolingo.com/hc/en-us/articles/204728264-How-do-I-report-a-bug- Writing it to us is really useless.