Two chairs in the room should also be accepted. "A" vs. "The" is a recurring problem throughout the course. Since Ukrainian has no articles, the two words should be interchangeable. Reported.
In Ukrainian it can be either, since we skip the "to be".
Can be "not a sentence" (a phrase? a combination of words? how is it called in English?), "two chairs in the room". Can also be a sentence "Two chairs are in the room" (Два стільці (є) в кімнаті, we skip є)
I would like to translate this sentence as "This room has two chairs." or "The two chairs are in the room." Doesn't it need to be a complete sentence if it has a period at the end? Without the period it could be a sentence fragment. (I think that is the term you are looking for, by the way)