uncharted: (adjective) describes a physical area that has not had a map or survey done of it; used figuratively to talk of areas that are new to the person navigating them
Used in the expression uncharted territory.
unchartered: (adjective (uncommon, at least in its correct sense) describes an organization that does not have a charter or written constitution
NOT used in the expression uncharted territory.
Chartered (adjective) can mean to have a charter or constitution, but it can also be used to describe a vehicle that has been rented, usually with the driver, for private tours. If the vehicle was not chartered, you would say “not chartered” rather than “unchartered”.