iPad 8th Gen won't update using Apple Devices App (Win10)

Have you attempted to perform the update directly from iPad settings? Perhaps you have a beta profile installed?

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.

I had precisely the same problem as you. I was getting quite frustrated, as everything I tried didn’t work. Then I stumbled on some advice that actually worked. Delete the app file, then download it again.

To do this:

Settings -> General -> iPad Storage.

Then find the update in the list, tap and delete.

Then download the update again (Settings -> General -> Software Update etc) 

It still came up with the message saying there wasn’t enough space. After a short while, I realised that it was installing. Unfortunately, I can’t be more specific about anything I might have done then, as I was VERY tired and quite frustrated. It’s possible that I may have tapped on the update file, as I was thinking of deleting it again, but I can’t be sure. I certainly didn’t do anything major.

You may find this useful, or you may feel that the vagueness at the end is annoying. I am a scientist, so I appreciate the need to be clear and precise. Unfortunately, it’s an experiment that I cannot repeat to replicate the outcome. All I can say is that I now have 18.2.1. By the way, it was on an iPad 7 Gen, so I think your 8 Gen should be OK. Good luck!