I assumed that the "Tu" meant that a perhaps a parent was allowing his child to order fries. In English the "may" can be substituted for "can" when permission is being given so I wrote "You may order fries".
My idea was a mother telling her little kid at a snack bar: 'You can ask for fries'. But then Duo, the evil father, says 'No!!!!!'.
That would just be "Tu commandes des frites". "Peux" in this sentence means "can" or "may".
French fries. Strips of potato deep fried until crispy on the outside and fluffy on the inside and then lightly salted.
Why can't it be 'les frites'? Presumably a menu is being consulted and 'frites' appears on it so it can be understood as 'the fries that appear on the menu'. If I was ordering I'd say 'the fries' ( or rather, bring British, 'the chips'). Definitely a blind spot of mine: sometimes ascertaining the difference between using 'les' and 'des'.
I always see commander as to command, regardless of getting it wrong every time I see it I always happen to giggle at the thought of someone granting permission to someone else in commanding food.