There are two factors at work; the on going aluminum can shortage and the shortage of the artificial sweetener that's used in Coke Zero (in particular). People staying at home have driven up demand for soda in general.
There no shortage of aluminum just the production to make cans, two new plans are supposed to open sometime in 2021. In the mean time Coke cut back on Caffeine-free Coke, Cherry Coke, Coke Zero and Fresca to focus on their other brands.
I'm not sure if the artificial sweetener shortage is still as big of an issue, it was reported by Coke to its investors back during the early stages of the coronavirus that they were having a disruption in its supply chain and some elements of the product came out of China.