We talked about this above in the ingredient notes. But I thought it necessary to add here too. And yes, you totally can. In fact, if you're subbing, I recommend subbing with buttermilk. Reason why is buttermilk actually helps with the leavening process and may give you a fluffier biscuit. If milk is all you have, milk works too, and will likely give a richer flavor to the biscuits. It may also help add more moisture to the biscuits.