Sometimes, when we attempt to add an offer to an email address that's already received an offer previously, Kajabi returns this error.
The reason behind that is Kajabi sets up an external_user_id by default which is different from the email address. When attempting to add another offer to the same user, the external_user_id needs be the same. This can be avoided by setting the user's email address as the external_user_id so it's the same each time.
You can do this while setting up your Action step in Kajabi in the Zap Template like this: