A sharp curve was in front of them?
"A sharp curve was in front of them." is identical in meaning.
"In front of them was a sharp curve". was not accepted.
Is it because I missed the "there", "In front of them there was a sharp curve"?
Well now I have tried that as well and it still was not accepted!
I can add that (with "there" included), just be aware that this "fronting" in English would also correspond to the similar word order in Czech: "Před nimi byla prudká zatáčka"