Coupon is a premium feature on Weebly so only users with a Business plan can create coupon codes on Weebly Admin.
If you do not own a Weebly Business Plan, you can use Better Coupon Box to design a welcome popup like this one:
We have finished a blog post to show you how to do it on your end, please follow: 3 Best E-commerce Pop-ups (No coupon required)
>> Get your own coupon pop-ups that turn visitors into email subscribers, social followers and increase online sales. Get the app for FREE.