This is unlocked by reaching 70% Game Completion.
You can check your game completion by pressing the Y button on the main menu.
The things that contribute to game completion are:
- World Tour Stars
- Characters Unlocked
- GP Wins
- Staff Ghosts Beaten
- Stickers Collected
- Mods Earned
Winning GPs on Expert is a quick way to get game completion if you're starting to struggle a little.