That is an unhappy inconsistency of the course that has not yet totally been fixed.
Just remember that you have an imperative adressing one person Go! Eat! Sleep! which in generally in Swahili is equally to the infinititive verbform without 'ku-'.
kulala >Lala! Sleep! kuimba > Imba! Sing! etc.
There are some exceptions to be memorized like Lete! Bring! Njoo!, Come!...
Now if you want to adress more than one person/a group of people you use a different verbform in Swahili. You basically change the last letter from a>e (if the last letter is other than a it remains unchanged) and add - ni
kulala >lala >lale>laleni
Now you are telling a group of people to sleep. In English however in the same setting nothing changed. You would still say 'Sleep!' regardless the number of persons you are talking to. For this reason this course tries to use 'you all' in the latter case to indicate we are addressing more than one person. Unluckily this principle has not yet been applied to every example, which causes at times a correct answer to be marked as wrong. We have to be patient and keep reporting. There have been so many things already improved on this course, the rest will follow.