My 2023 Nissan Patrol Y62 automatic won't shift into sixth or seventh gear automatically

If your can manually force the shift then there’s probably nothing wrong with the actual gearbox and its mechanicals. Instead, the problem is likely to be an electronic one, perhaps with the speed sensor that tells the computer the car’s road speed and tailors the gearshifts to reflect that. Don’t forget, however, that gearboxes like the Nissan’s with many gears, often won’t select higher gears at low speeds. So if the car refuses to select sixth or seventh gear at urban speeds, that’s possibly just the calibration Nissan has fitted to it.

If this is an actual problem (and not just a characteristic) this is a very new vehicle and would be covered for this sort of problem by the factory warranty.