After completing the main game, players can unlock time trials which is available in every version of the game across platforms except one, the Nintendo Switch. Once completed a prompt should appear when you are selecting a level to play, giving you the option to choose between normal play or time trial. The Switch version does not have this option, yet.
Sumo Digital have confirmed that Time Trials will be coming to the Nintendo Switch version some time in the near future as stated by Design Lead Brad Davey via NeoGAF:
“I thought I’d out myself to pass on some good news; I can confirm that we’re working on getting Time Trials patched in to the Switch version of the game. I can’t tell you exactly when this is going to happen, it’s not quite as simple as flicking a switch, there is some work on our end involved on getting this up and running. As soon as I have more news I’ll be sure to pass it on.”