"There are very few inexpensive apartments here."
Translation:Levných bytů je tady velmi málo.
Because it is very strange to stress it like that except some special occasions. It is actually possible in sentences like "Velmi málo levných bytů je tady." which is still unlikely to be used in practice, but at least makes sense.
Remember, you should report the complete sentence that was rejected.
In normal usage, it sounds just like "It is very few cheap apartments here" which is just as incorrect in English as it is in Czech.
However, it could be used in Czech with a very particular (incredulous) intonation, perhaps with a "?!" at the end, meaning "Do you really want to say that there are very few cheap apartments here?!".
TU is another way to say TADY. Another acceptable sentence is TADY JE VELMI MÁLO LEVNÝCH BYTŮ.
Your sentence was marked wrong because the verb TO BE would not be stuck at the the very beginning like that. Either you put the HERE there or the very few apartments there but not ARE.