iPadOS 16.1.1 error stating "Unable to install update"

Begin by ensuring that you don’t have beta profile installed. If a beta profile is present, installation of production software updates may be inhibited. This support page provides the necessary information to remove the beta from your iPad:

How to uninstall iOS beta software - Apple Support

If the iPadOS 15.7.1/16.1 update still doesn’t appear, perform a forced-restart of your iPad:

Force restart iPad (models with Face ID or with Touch ID in the top button)

If your iPad doesn’t have the Home button, do the following:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button.
4. When the Apple logo appears, release the top button.

Force restart iPad (models with the Home button)

Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.

After the iPad restarts, look again for the update.

If your iPad is capable of being updated to a more recent version of iOS/iPadOS than is currently installed, these support pages outline both the available methods by which the update can be performed - and troubleshooting should you experience difficulties:

Update your iPhone, iPad or iPod touch – Apple Support

If you can't update or restore your iPhone, iPad or iPod touch – Apple Support

If your iPhone, iPad or iPod touch won’t update - Apple Support

iOS update and restore errors – Apple Support

Be aware that while an iPad may be capable of update, in some circumstances you may need to use a computer to perform the update. To do so, you will need access to either a Window PC (with iTunes for Windows installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of MacOS). Ensure that your installed version of iTunes has been updated to the most recent version; the essential device drivers are included.

Download and use iTunes for Windows – Apple Support

Update to the latest version of iTunes – Apple Support

Update software on iPhone, iPad or iPod touch in iTunes on PC - Apple Support

You should note that for your iPad to install an available update, its battery must be charged to at least 50% capacity - and/or be connected to its Power Adapter. Similarly, the iPad can only be updated over a standard WiFi connection - or be updated by a PC or Mac using iTunes or Finder - over a USB connection.

Is your iPad using a VPN? You need to, temporarily, disable the VPN.

Is your iPad running an iPadOS Beta version? You need to remove/uninstall the iPadOS Beta software before you will be able to install the Apple “officially” released version of an iPadOS upgrade/update.

Also, make sure your iPad is charged to, at least, 50%, or greater, before attempting the iPadOS upgrade/update.

Are you trying to use a Wifi hotspot through a smart phone using a cellular service?

This will not work.

Apple does not allow iOS/iPadOS upgrades/updates through usng a cellular service network as an Internet download/install source.

Apple only allows iOS/iPadOS upgrades/update using a dedicated wireless, Wifi connection network through a WiFi MODEM/router box that connects to an local ISP ( Internet Service Provider ).

If you have no such dedicated home wireless, WiFi network setup, you will need to seek out outside locations that offer free WiFi through such a service to do Apple mobile device iOS/iPadOS upgrades/updates.

OR

Search online to locate and call contact your nearest, “local” Apple Store location to try/attempt to make an appointment to get your Apple iPad upgraded/updated to its latest iPadOS 15 version.

OR, alternatively, use the “Get Support” link, in the top black menu bar, at the upper right hand corner of every Apple Support Community Webpage to attempt to schedule an appointment with your nearest Apple Store location.

Apple Store employees will be glad to help/assist you in upgrading/updating that iPad for you.

Best of Luck to You!

Does the error mention anything else besides "Unable to install update"?

for example:

Try the troubleshooting here: click here ➡️ If your iPhone or iPad won't update - Apple Support

Do you have enough storage space? You need around 6GB free to install it. Check Settings->General->iPad Storage

Then make sure you are connected to a good strong Wi-fi network, and that it's not restricting access to any Apple website.

Try a different Wi-Fi network if at all possible.