I was trying to test coupon codes and just couldn't get them to work. I created this video which I hope helps explain what I'm doing.
Thanks for reaching out to us and sharing the video.
To clarify your doubts,
The coupon ID acts as the coupon code in Chargebee. The coupon ID has to be entered by the customer or by you for availing the coupon discount.
Regarding the "Not Applicable" error that you're seeing on the coupon page, this is related to the Webhook configuration. A webhook is configured so that whenever any event occurs at your end, the event details get sent via the Webhook to your server. This is only required if you want Chargebee to send details to an external system, for a code/API-level implementation on your end.
Note that this configuration would not affect the coupon in any way. You can ignore the webhook settings if you don't require it and still use the coupon.
Hope this helps.