There may be an issue with the server or the credentials being entered. Please check to make sure the correct email or phone number is being entered. If you are in the linking stage, and it does not link, that is a direct response to incorrect information being entered.
Open the Feit App and go to the Profile section on the bottom right-hand corner, once selected under the tap to set name, you will see your login using email or phone number. If this does not resolve the issue please contact Feit Electric customer service.