Flour is in the grains food group. That is because flour is made from wheat and wheat is a grain. That makes flour a grain product. Other kinds of flour, such as rice flour or rye flour also fit in this group.
The only other group that you could even imagine flour being in is the fruits and vegetables group. But wheat is not classified as either a fruit or a vegetable.
The other food groups are meat, dairy, and nuts and fats. It is clear that wheat does not belong to any of these groups.