Before you ask about Challenger Ahri in the Snowdown Shop...

Challenger Ahri is currently bugged via the snowdown shop and not allowing people with the skin in their snowdown shop to successfully purchase the skin. Riot is aware of the issue and is currently working on figuring out what's wrong, so be patient and wait it out. You may find more information here : https://twitter.com/RiotKateyKhaos/status/677298285699198977