Coupon is a premium feature on Weebly so only users with 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 Ecommerce Pop ups (No coupon required)
>> Get your own coupon pop ups that turn visitors into email subscribers, social followers and increase online sales. Get app for FREE.