Translation:One kilometer is one thousand meters.
If you didn't have a typo, then your answer should have been accepted and is worth an error report. Numerals are supposed to be automatically accepted for numbers, but I think I read somewhere that higher numerals (100+) are not automatically accepted, so the contributors might have overlooked that they needed to enter "1000" as an acceptable alternative.
I notice only one person above uses "metre" rather than "meter". Personally, I grew up with "metre". We converted to the metric system from imperial in the 1960s. I can tell you that it was a relief, after 12" (inches) per foot, 20/- (shillings) in the pound (£) and 12d (pennies) in the shilling. Also rods, poles, perches, furlongs, chains, roods and acres. Gallons, quarts, pints. All gone forever thank god!
Basically, the computer can't switch the order. It would mean that it has to store the information as "一キロメートル is the proper translation of 1000 meters" which is wrong. Stick to the order the sentence is given in.
Also, you will find instances where you CAN switch things in a sentence and Duo will be fine with it (there's almost always an exception to things). The reason I don't think it should be accepted here is because of the topic particle putting the focus/emphasis on "1 kilometer". Your sentence, due to the way English works, puts that same focus on "A thousand meters" which is not faithful to the original Japanese sentence.