An email already attached to another account is unavailable for addition to yours. This prevents one address from being used to claim multiple account identities. Signing in with it can lead to the other account rather than the workspace you intended.
Check whether you previously created an account with that address or used it through a social connection. Do not delete an account or team to solve the conflict without reviewing its resources first. Contact support if you cannot identify the existing account or need help with the access situation. Never send mailbox passwords, sign-in codes, or a private verification link.