In my room, was only a table with one chair. špatně?
Duo counts it wrong because it wants the word "there" in the sentence. With a different word order, you wouldn't have left it out - There was only a table with one chair in my room.
Indeed, see the correct form above.
Was or were? Table plus chair.
http://guidetogrammar.org/grammar/sv_agr.htm point 3