A coupon code or promotional code is mostly given to existing subscribers. Coupon codes can only be applied to monthly plans.
Steps to apply a coupon code
From Shopify admin, click on Apps and click on Debutify app.
In the Debutify app, click on Plans.
Confirm which Plan to apply the coupon code to.
Enter the code and click on Apply Code.
IMPORTANT: Just like a password, a coupon code is case-sensitive.
Carefully review your selection and confirm your agreement to your selected plan by checking the box and click on Start Plan.