This error can occur when a supporter record exists but, for some reason, isn't displaying. Typical causes include signing up on a double opt-in page without confirming the opt-in, and some odd situations where a page may be submitted "incomplete". If you have verified that the supporter in question does indeed wish to be added to your list, then you can use the following method to activate their record.
Use the "add a supporter" widget, on the Supporter dashboard, specifying only the supporter's email address.
This should force the record to go "live", enabling you to use the record. If, for some reason, this process does not restore the supporter record for you, please reach out to our support team and we can take care of it for you.