Sometimes only one tense is accepted in English, while there is no indication whatsoever of the correct tense in Indonesian. It seems completely arbitrary, why in this case you accept past tense translation only.
It's just because no-one has had the time to enter in all the possible variations in tense and number yet. I'm surprised you got this far though the course without being tripped up by that over and over again. I know it's been a problem for me since about lesson 3