"The men and the women walk."
Translation:yIt loDpu' be'pu' je.
The plural markers are optional in Klingon, so yIt loD be' je, could be interpreted as either, "The man and the woman walk," or, "The men and the women walk." Sometimes the context of the sentence (i.e. the conversation we've already been having) makes it clear which was intended, but you can make it clear you mean the plural by using the plural markers. This sentence was created in hopes you would practice the plural markers, but you are not wrong to write it without them.
You seem to have missed the fine print.
This sentence belongs to the "Basic sentences 1" unit, and in the tips and notes for that unit -- https://www.duolingo.com/skill/kl/Sentences-1/tips-and-notes -- there is a section on "Plurals" which says that
Plural suffixes in Klingon are optional and can be left off. Later on, you may come across sentences where only the grammar tells you that a noun must be plural.
If you have not been reading the tips and notes regularly before starting a lesson, I encourage you to do so.
Do you understand the terms, "subject", "verb", and "object"? In Klingon, the "object" (the person/thing the action is done to), if there is one, comes before the verb. Then the verb (the action or state, with an appropriate prefix). Then the "subject" (the person/thing doing the action).
For this sentence, there is no object, so the sentence starts with the verb (yIt - "walk"), followed by the subject (loDpu' be'pu' je - "the men and the women"). Within the noun phrase, "the men and the women," the order is flexible and you could mention the men first or the women first. Likewise, in the Klingon noun phrase loDpu' be'pu' je, you could list either loDpu' or be'pu' first. Because there may be a reason why one is being put before the other, we ask that when you translate for Duolingo you keep things in the same order in both languages as much as possible, unless there is a grammatical reason why the order must be changed. Thus, in this exercise, we ask that still list loDpu' before be'pu', but the grammar requires that you put them after the verb.
These details are explained in the Tips & Notes, but I know it’s a lot of new information and the differences from English can make it difficult. However, since Duolingo has hidden the Tips & Notes I want to make sure you know about them and where to find them. If you have not been reading the Tips & Notes, I would like to ask that you review those so we don’t have to repeat too much of the information that we have explained there.
If you are doing the course on iOS or Android, you cannot currently access the Tips & Notes through the app. To access the Tips & Notes, you will have to access the course using a web browser at https://www.duolingo.com/. You can still do it on your mobile device, but you will have to use the web browser instead of the app (or you can do it from a computer). When you click on a Skill, it will expand to reveal a Start button, a key, and a light bulb.
If you click on the light bulb it will reveal the Tips & Notes and give you a detailed explanation of the grammar that is introduced in that Skill. If you have questions after reading the Tips & Notes for any Skills, then please return to the forum to ask your question, explaining what you didn’t understand or what seems contradictory to you.