This can occur when the Primary Email addressed in Sites & Settings is mapped to an RE phone type that is not an email address.
To resolve this:
1) Login to Blackbaud NetCommunity as a supervisor user
2) Navigate to Administration - Sites & Settings
3) Scroll to Phone Type Mapping and verify the Primary Email Address, and the RE mapped value
4) Login to RE as a Supervisor User
5) Go to Config - Tables - Phone Types
6) Edit the mapped Email Address and change the phone type to Email Address