Because the course is still in the beta phase and many possible sentences haven't been added yet. This sentence should be accepted, please report it.

Yes, fölött a nagy város is just as wrong in Hungarian as "the big city above" in English.

"above" is a PREposition in English which comes BEFORE its noun; fölött is a POSTposition in Hungarian which comes AFTER its noun.

Hungarian adpositions are (almost?) all postpositions which English uses prepositions (with few exceptions such as "ago").

So words such as között, fölött, mögött etc. come AFTER the nouns they refer to.