Why can't I see the "Extra" content?
Try checking the "Secrets" menu by hovering over "No Save" in Mania Mode.
Based on the number of medals you've unlocked (17), I don't think you've unlocked any content in the Extras menu yet. Nintendo Life claims you need at least 20 medals to unlock Mean Bean Machine.
To clarify, Sonic Mania has two different menus for unlockable content: Extras and Secrets
Secrets Menu
- (1 medal) Super Peel Out
- (7-ish medals) Insta-shield
- (12-ish medals) & Knuckles Mode
- (16 medals) Debug Mode
Extras Menu
- (20 medals) Mean Bean Machine (Source)
- (24 medals) D.A. Garden
- (32 medals) Blue Spheres
For more information, see Is there a reward for beating the Blue Sphere bonus stages?