After my Dartrix evolved, I decided not to learn Spirit Shackle, but now I can't use Decidueye's Z-Move. I didn't save my game yet, but if I restart it, I will lose about 2 hours of gameplay (I will start saving more often now). I also do not want to trade it (too risky, and I spent a long time training it). I have not unlocked Poni Island yet, and I only have the first two areas in Ula-Ula Island. What can I do?
EDIT: I decided just to move on, and just use Grassium-Z until I beat the Pokemon League. Thank you for your answers!