PSA: If a champion/skin is on sale in the Store, you cannot buy it in Your Shop yet

You will be unable to buy a skin in Your Shop if that skin is on sale in the Store currently.

Since they come as a combo champ/skin deal if you don't own the champion, if that champion is on sale in the Store you need to buy the champion separately first (IP or RP), or wait for the Store sale to end.

This is to prevent you from making a bad purchase unknowingly. If the Your Shop deal is greater than the one in the Store, you can simply wait for the item to go off sale.

Check the Store Sales news articles here to see what's currently on sale.