1. Open the coupon and set Type to Percentage of Cart (Subtotal) — this is the only type that supports exclusions.
2. Switch on Exclude Meals from Coupon. An Excluded Meals search box appears ("Search meals…"); type at least one character and pick each meal to exclude.
3. Under Allow on, only À la Carte and Catering remain selectable — the other ordering systems are switched off automatically.
4. Save.

At checkout the excluded meals are simply left out of this coupon's discount; other items still get the percentage, and any other coupon in the cart still discounts the excluded meals. If you later switch the toggle off, the saved exclusions are dropped.

Saving fails with "Exclude Meals from Coupon only works with the Percentage of Cart type." if the type is wrong, and this option cannot be combined with Apply Discount to Recurring Subscription Rate (enabling one turns the other off).