I don’t know why you “can’t,” but you honestly don’t need to. With other water bottles I would fill them with only ice and by the time my kids got tonsoccer and had their first water break, the ice had melted enough for ice cold water. With this one, I put some ice and mostly water and it’s (their words) “freezing cold like a glacier!”
I am not sure that you “can’t,” but you don’t need to. You just fill it with ice and then pour in the water. We did this and did a test to see how long the ice would last, and over 24 hours later there was still ice in there. We love it and bought a second.