Did you try a hard reset?
1.
Press and hold down the iPhone's Sleep/Wake button on the right side of the handset.
2.
With the Sleep/Wake button still held down, press and hold the Home button on the front of the handset.
3.
Continue to hold both buttons while the display remains blank, until it comes back on with the logo
you can also try this :
Remove and download the update again
If you still can't install the latest version of iOS or iPadOS, try downloading the update again: Go to Settings > General > [Device name] Storage. ... Tap the update, then tap Delete Update. Go to Settings > General > Software Update and download the latest update.