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.
if you put a semicolon, you must say "there", without semicolon, it should be acceptable. if you dont want to use "there" You can say as well "only a table with one chair was in my room."
Was or were? Table plus chair.
http://guidetogrammar.org/grammar/sv_agr.htm point 3