Alright, lets get started.
Line of symmetry means a line which parts the figure in two equal parts.
In another words, one part would be reflection of other part.
In American flag, we could see, 4 incomplete and 3 complete lines, none of the line is parting the rectangle in equal two parts.
hence there is no lines of symmetry is there in one red rectangle on the American flag. : Answer
Hope it will help :)