i cant buy hextech sion in my snowdown shop

There's something with the Snowdown Shop that prevents you from buying the skins if:

1.) The skin is currently on sale in the normal store

or

2.) The champion for the skin is currently on sale in the normal store (this only applies if you don't own the champion)

Right now, Hextech Sion is on sale for 50% off in the normal store, which is why you are unable to purchase it in your Snowdown Shop. You can either buy it right now for 50% off in the normal store, or you can wait until it is no longer on sale (which is Dec 30th) to buy it from your Snowdown Shop for whatever discount you have for it.