"The soldier must not sleep."
The most common cause of that is misreading the I (capital i) as a l (lower case L). Note that you can tell the difference on Duolingo by the small curl at the bottom of the l (lower case L) that is not present at the bottom of the I (capital i). The sentence in this exercise has an I (capital i), not a l (lower case L). However, there are other possible problems and for us to figure out exactly what is going on, it is helpful to see an image of your rejection. If it happens again, please take a screen shot and send us a link to the image so we can figure out what might be going on. Thanks.