When you add a player and they are 13 years old or older they will need a unique email address. If their email address is already in use, then you will need to use a different email address for this player.
Athlete One support can assist the player if their email is in use with another account by removing that account or updating the email address associated with it. If the player needs further assistance, advise them to submit a support ticket with the request to cs@athleteone.com