Hello there, in order to be able to use the Cut Ocarina you have to obtain the HM Cut in the region, in your case, it's obtained in the Ilex Forest once you've caught all the Farfetch'd.
Hola, para poder usar la ocarina de Corte debes obtener la MO Corte en la región correspondiente, en tu caso, se obtiene en el Encinar tras capturar a todos los Farfetch'd.