A team must keep a seated owner. Removing or demoting the last owner, or leaving while you are that owner, would leave the workspace without someone authorized to manage it. The protection is intentional.
Arrange another owner's membership before attempting the change. An unaccepted invitation does not replace a seated owner. If ownership cannot be arranged because someone lost account access, contact support with the team and ownership situation. Do not remove unrelated members or share an owner's credentials to bypass the protection. Deleting a team is a separate action with its own consequences.