This is the same problem people had with the Snowdown Shop. Basically, there are 2 reasons why you would be unable to buy a skin from the Lunar Revel shop:
1.) The skin you want to buy is currently on sale in the normal store,
or
2.) The champion of that skin is currently on sale in the normal store (this only applies if you don't own the champion).
Since you don't own Zed, and Zed is currently on sale in the normal store, you are unable to make the purchase. To fix this, you can either:
1.) Wait until Zed is no longer on sale, which will be at the start of February 2nd
or
2.) Purchase Zed right now for 50% off (or with IP).
Either of these options should allow you to make the purchase. I recommend waiting until February 2nd if the discount you have is greater than 50% so that you don't spend as much RP.