You can't use flour for thickening. Flour we all know is specialized in pasta, pour water in, the flour will be completely fused with water, it becomes a dough, it will not become liquid, able to adhere to the surface of the food, so the thickening of flour is certainly not successful, no matter how much water, it is hot, it will become a block, so it must be used with starch to thicken the gravy.