"July is a month of summer."
Translation:Lipiec to miesiąc lata.
But they are accepted in every "Noun is noun" exercises.
Either one or the other has been put as the 'default' Polish sentence, that's just a choice, but then the other is also correct. So my guess is that you keep making some small typos and get corrections with the 'default' sentence, which unfortunately happens to use not the construction you wanted to use yourself.