Check this Announcements support page to see when they are done distributing Mystery Champions/Skins
Announcements - Riot Games Support
They distribute these things in waves. Not everyone will receive theirs at once.
They should make an announcement when all rewards have been handed out to those who are eligible. After that, if you still don't have one and you think you were eligible, you can submit a Support Ticket.
Be patient. If you earned your reward you will get it eventually.
If you have questions about eligibility, you can check the announcement here.
