If you don’t have popsicle sticks (the flat wooden variety), then you could use lollipop sticks (which tend to be rounded) instead. The benefit of using something like flat wooden popsicle sticks is that they’ll tend to fit better inside the brownie squares with less likelihood of the brownie pops breaking apart. With that said, if your brownies are a decent thickness then you shouldn’t encounter a problem regardless of what you use.