Update downloaded but get an error message trying to install

If the update has been downloaded - but will not install - perhaps you have insufficient spare space on your iPad?

Settings > General > iPad Storage

Your iPad must also be charged to at least 50% - or must be connected to its Power Adapter - otherwise the system update will not be attempted. Note: Updating your iPad whilst connected to the Power Adapter is always recommended; doing so avoids any possibility of a battery issue causing unrecoverable failure of the update process.

A further possibility is a corrupted download. This can be easily resolved by deleting the the corrupted update...

If it exists, delete the failed update package from the iPad...

Settings > General > iPad Storage

Scroll down the list of Applications until you find the Apple Update - then tap and select Delete Update.

Next, force-restart your iPad:

• On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On an iPad with Face ID: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.

After restarting the iPad, attempt to update the iPad:

Settings > General > Software Update

I hope this information and guidance proves to be helpful in resolving the problem.