The functionality requires the Coupons add-on to be installed and enabled.
A store admin can configure membership-specific coupons via the Coupons section of the store Admin area (Discounts > Coupons).
To configure a coupon that only members will be able to use:
First, open the Coupons section of your store Admin area (Discounts > Coupons) and click New discount coupon:
Next, scroll down the coupon details page and locate the Membership field.
Click on the field and select the applicable membership value from the drop-down.
Save your changes.
If a non-member uses a members-only coupon at checkout, they will get an error message like the following:
Sorry, the coupon you entered is not valid for your membership level. Contact the administrator.
You can find mode details about coupons in the section Coupons Add-on.