The direct explanation of the error content is:
A coupon with this offer code already exists, please use a different Coupon Code.
reason:
1. The listing is already doing coupon activities, and multiple coupon activities cannot be performed at the same time;
2. Coupon campaigns cannot be assigned across accounts. For example, if the campaign has already assigned the listing of eBay account A, the listing of eBay account B cannot be assigned to it;
3. The coupon code has been used before and cannot be reused.

Solution:
Create a new coupon campaign and assign it to the listing of the same eBay account.