As an administrator of an administrative unit, you can reset passwords only for users who are assigned to your administrative unit. Make sure that the user whose password reset is failing belongs to the administrative unit to which you've been assigned. If the user belongs to the same administrative unit but you still can't reset the user's password, check the roles that are assigned to the user.
To prevent an elevation of privilege, an administrative unit-scoped administrator can't reset the password of a user who's assigned to a role with an organization-wide scope.