If a user is getting a message such as this one:
It means that the coupon code that you set for the guest has an email restriction. To fix the issue, go to that coupon edit screen and remove any email restriction from it.
Also, if you did not enable the option for existing users to get discounts on the general screen and they try to make a purchase via referral code, the coupon will be removed once they are on the checkout page as it will only work for new users.