Ash here from the Shopify team. Welcome to the Shopify Community!
I see you're running into the error message "ERROR: ORG.OPENID.APPAUTH.GENERAL -15" while trying to access the Shopify app on your iPad. I've seen this error message in the past, and I have a couple steps for you to try that should clear this up.
Open your iPad settings and scroll down until you find the Shopify app. Tap on the app to open the general settings for it, and toggle the option "Remove all sessions on next launch". Open the app, and click "Log in", and you should be able to log in without further issues.
If you still experience the issue after turning on that setting, try toggling on or off the automatic timezone setting in your iPads general settings and giving it another shot.
Let me know by replying here if this resolves the issue you're facing! I'll be happy to help take a closer look if the problem seems to persist.