ReferralCandy does not manage coupon code creation/deletion for you. Hence, for all coupon codes used in your referral campaign, you have to make sure to manually create them in your store's admin panel.
If the advocate is not able to apply the referral reward coupon code, here are some steps you can take to troubleshoot it:
Make sure that the friend offer coupon code was created in the first place. You'll generally create this in your store's admin panel, so check in there to see if it exists. If it doesn't, please create the coupon code so that referred friends are able to use it as part of your referral program.
If the friend offer coupon code was previously created, check the expiry date to see if it has expired. With ReferralCandy, you are able to specify an expiry date for the friend offer coupon code, and hence, if you initially created the friend offer coupon code to expire on that date, friends will no longer be able to use that particular friend offer coupon code. Related: Learn what happens when a friend offer expires here.
If the friend offer coupon code was created, and is not expired, check the conditions that you have put in place for the use of the coupon code. It may be that the referred friend's purchase does not fulfil the conditions to be able to apply the coupon code to their order or maybe it isn't a multi-use coupon code.
If all else looks ok, please contact your shopping cart platform vendor to troubleshoot this further.