"Nevím, ve kterém pokoji jsme byli."

Translation: I do not know which room we were in.

[Stephen900455]

"I do not know in which room we were" is correct grammatically and is often said -- I recommend it be accepted as an alternative

[BoneheadBass]

It is grammatically correct, or at least it was for quite a long time. In these days of more relaxed grammar rules, I don't know if the traditional ones still apply

FWIW, where I've lived (US East Coast/Mid-Atlantic areas), that construction would rarely be used in everyday speech, even if it is still grammatically correct. It sounds bookish or affected. I can't speak for other US regions or usage in BrE, however.

[Stephen900455]

I hail from the same area. As Duolingo accepts multiple translations, i see no reason to not include this one. I can assure you there are many of us who do speak this way.

[BoneheadBass]

"I do not know in which room we were"... (a) is grammatically correct in English, (b) has multiple user reports, and (c) is a literal translation of the Czech original I've added it as an acceptable translation.