This means an account with that email already exists. This often happens when someone shared a BIP or course with you before, or you signed up with Google.
What to do
Go to the log in page instead of signing up.
Try Sign in with Google if that email is a Google account.
Otherwise click Forgot your password? to set a password.
If you were invited to a team and the invite says "Invite cannot be sent to this email", that email is already on another team. Ask your admin to contact [email protected].