I assume the Kiosk mode is "the guilty" for this. Kiosk mode actually blocks all normal processes like app udates etc. very effectively. Please disable the Kiosk mode and then finish the iOS update > deploy Kiosk mode back to the device.
More about iOS update here: https://onlinesupport.miradore.com/hc/en-us/articles/360000313765-Software-update-for-iOS
Hopefully this helps you out.
- Jarkko / Miradore Online Support