"They are cooking tonight."
Translation:De lager mat i kveld.
kokkererer = kokkelerer. However I think Ingebj agree with me in the strangeness of kokkerere. Kokkelere is the version I'd prefer myself (I wrote them in the infinitive, add an r to get to present). These are just the acceptable answers however, the "correct" one is De lager mat i kveld. If you give Duolingo an answer that is different enough from the (usually) single correct answer, it will suggest stuff from the list of acceptable answers. We don't teach people kokkerere or kokkelere but they are allowed to enter as answers.
Here, like everywhere in the course, it is a good idea to learn the most standard translation first and leave the stranger words for later (to avoid spelling mistakes and so on), I do agree. Sometimes the allowed answers seem to be rather far from the standard translation (based on what I have seen in the discussions in the French and German courses mostly).
Are you sure it didn't say "kokkelerer"? Which would be a word we use to describe the act of cooking (usually a more elaborate meal, at least more than boiling an egg). Mostly it would not be used to describe what a professional chef does, but rather what you do at home as an "amateur chef".
Not incorrect, but it would change the meaning to one that probably isn't intended.
In English, "tonight" is often used about the (late) evening. It may be dark outside, but it's not necessarily bedtime yet. In Norwegian, "i natt" would be understood as the part of the night when you're expected to be asleep rather than cooking.