I guess you have deleted the coupons.
Go to: yourdomain.com/public/admin/coupons and see all the coupons.
Note: if you delete a coupon, you will be redirected to the "All Coupons" listing page, and if you click on the back button, you might see that error. But there is nothing to worry about.
If you have deleted the coupon then obviously it cannot be applied by the customers.
If the coupon exists, then check the expiry date of the coupon.