"Even though I created a coupon code, the button 'I have a coupon code' is not appearing during checkout."
By default, coupons are not available to every membership.
To fix this, you'll need to:
Head over to the coupon page.
Select a coupon.
Check the box next to each membership & save.
The next time a member tries to sign up, they should see the "I have a coupon" text appear.
Below is a quick video guide.