If you receive an error that a user already exists but you don’t see them in your Users list they may be disabled. To locate disabled users:
1) Head to "Settings" in your menu bar.
2) Select "Users."
3) Click the checkbox near the upper right that says “Show disabled accounts”.
4) Proceed to the search bar and locate the user by name, location, email, or ID.
5) If you wish to enable the user, click their profile and change their Account Status to “Active”.