Some players reported that not switching focus from the installing app helped them with similar problems. If this won't work for you, just use the desktop version.
Viewing post in Yorna: Monster Girl's Secret comments
After restarting my phone, it eventually worked... BUT, I still can't play it; it crashes as soon as the intro cutscene ends. Consistently; have tried it 3 times. It always breaks in the same spot, right in front of the destroyed castle which I'm assuming is where I'm supposed to be getting control of my character.