Thank you for bringing your issue to the Help and Support Forum! Not being able to use a skin you just recently purchase can be an absolutely frustrating experience! Hopefully we can get to the bottom of this immediately!
After reviewing your thread, I went ahead and performed an in-depth audit of your account. From this audit, I was able to confirm that you did in fact purchase this skin! Furthermore, I went ahead and refreshed this skin's availability on your account. This should do the trick, and allow you to select it normally in Champion Selection!
If you are still unable to select this skin, please do not hesitate to let us know by creating a new Support Ticket* so we may work with you one-on-one until arriving at a Full Resolution!
* Keep in mind that this advice was given based upon the information provided. If you would appreciate a further one-on-one interaction to solve this issue, make sure to send us a Support Ticket! You can send one from here: https://support.leagueoflegends.com/tickets/new Also, if you could please provide a link to this forum thread in the new ticket, it would be incredibly helpful!