I get S - but i dont get a chest!

Have you earned a chest on that champion already this season? You can only get one chest per champion per season (this hasn't reset yet)

Do you have chests available to earn? You have 4 slots which you accumulate one spot every 7 days, so can only get a max of 4 at once before having to wait for more.

You can see your chest slots in your profile by hovering the chest icon, and in your Champions page in your Collection a champion you've already earned your chest on this season will have a gold border.