While trying to update my iPad software getting error as' unable to install'

In general, to update over the air from the iPad all of these requirements must be met:

• You need at least 15 GB of free storage (if you don’t have enough it will tell you)
• You must be connected to a Wi-Fi network 
• The iPad must be charged to at least 50%, and preferably plugged in so it stays above 50% during the update
• The iPad must not be in Low Power Mode
• The iPad must not be connected through VPN, and it may be necessary to delete any VPN apps and VPN profiles
• You cannot still have a beta of a previous version installed 
• You cannot have a previous version that was downloaded, but not installed - go to Settings/General/iPad Storage to look; if you find an iOS installer, delete it

If you can't meet all of them you can still update using your computer.

If your iPad is capable of update 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 Windows PC (with iTunes for Windows installed - or the more recent Apple Devices App) 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

Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

Apple Devices App (Windows PC):

https://apps.microsoft.com/detail/9np83lwlpz9k?hl=en-ms&gl=MS

(*) If you don’t have access to a suitable computer of your own, that of a helpful friend or family member will suffice. Alternatively, the technicians at the Genius Bar of your local Apple Store or Apple Authorised Service Provider (AASP) will be delighted to assist.

Genius Bar Reservation and Apple Support Options - Apple

If you have a beta profile installed, ensure this has been removed from your iPad - as, otherwise, 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

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.