Can I make a coupon that only works at lunchtime?

Not as a coupon. A coupon under Marketing → Coupons has a start date and an expiry date, a minimum order, a usage cap and exclusions, but no hours. For a time of day use Quiet-hour offers under Marketing instead: an automatic percentage off every order placed inside a window you choose, with no code to type. Both times are inclusive and are judged in your restaurant's own timezone. It shows on the customer's receipt as Beat the rush. If you want one dish cheaper at lunch rather than the whole order, that is a happy hour rule under Marketing → Offers. The Promotions & marketing tab in the docs covers all three.