Again, my comment is on the discretionary use of articles in the translation. Sometimes, the translation says I'm wrong for not using "a" or "the." Sometimes, like this time it says I'm wrong for using it. Please adjust the lessons so that they consistently allow for either using or not using articles.
Now it does not accept "he cooked food" and asks for an article. In English, I would never say someone cooked a food. It's not a direct correspondence, even though "chakula" is singular. I always report, but I think comments may be helpful for the administrators to understand why people are reporting an issue? Though I'm not entirely sure how it works...
I have come to realise that our use (or not) of articles in English is subject to complicated rules which I, a native speaker, admit to not fully understanding. I just have a very firm view of what sounds right (or wrong). 'He cooked a/the meal' is fine, and so is 'he cooked the food', while 'he cooked a food' sounds awful.
I think it's because food is a collective noun (or mass noun), similar to water, soda, people. You can say "the" with these but not "a." They can also be used without an article. It's tricky, though, because some such nouns can also be singular. At least the one that comes to mind is "fruit." You can have a basket full of fruit, or eat a lot of fruit, but you can also have a fruit. Or a (bottle of) soda or water. That's my best explanation, though it's been a while since I studied grammar. I, like you, often go by what sounds right.