"Anoche nosotros cenamos arroz."
Translation:Last night we had rice for dinner.
It might sound archaic and stuffy to many people, but it's grammatically correct.
I would expect "we ate rice for dinner" to be accepted, but you have to mention dinner or dining in some way.
Most likely you would need the verb comer for that sentence. The verb cenar is, as far as I have seen, used more specifically to describe dining or eating dinner. Even though "last night" gives the context that it probably was dinner, I think Duo still would not accept the different verb.
the answer given here is what I wrote, I was told it should be "for tea" not for dinner!!