First of all, be patient. If you added the flour with the beef fat, it will thicken. The sauce needs heat to thicken, and it needs to be done slowly so you don't boil the milk. You can speed it up by whisking to incorporate and also allow steam to escapes, reducing and thickening the sauce. Increase the heat if it's taking too long but keep an eye on it and whisk. You don't want to scorch the bottom or boil the filling once you've added the milk.