"This email already has a Martialnote account. This person can't be invited."

Where: Settings › Roles and access › Invite an instructor (owner only).

Why: this email already has a Martialnote account: a member of this school, of another school, or an account whose school isn't created yet. In V1, a person belongs to one school only, and an account is never overwritten. For privacy, the message doesn't say which case it is.

What to do: if the person is already in your school, they sign in with their email from the sign-in screen. Otherwise, invite them with another email.