I am unable to update my IPad OS

This is because your iPad is very far behind and your not able to download that version anymore. The only way that you can get that update is to connect your iPad to a computer with the most current version of iTunes installed on it and let it do the update that way. If you connect it to a computer with iTunes and follow the directions you will be able to get iOS 12 but that is the only way that you are going to be able to get it that way. That's why under the Download and Install button it tells you to connect to iTunes.

Update your device using your computer

If you can’t update wirelessly on your device, you can update manually using a computer that you trust. If your computer is using Personal Hotspot on the device that you’re updating, connect your computer to a different Wi-Fi or Ethernet network before you update.

1. On a Mac with macOS Catalina 10.15, open Finder. On a Mac with macOS Mojave 10.14 or earlier, or on a PC, open iTunes.
2. Connect your device to your computer.
3. Locate your device on your computer.
4. Click General or Settings, then click Check for Update. 
5. 
6. 
   
7. Click Download and Update.
8. If asked, enter your passcode. If you don't know your passcode, learn what to do.

If you see error messages on your computer or need more space to update

Get help if you see error messages while updating your device with your computer. Or if you need more space for an update, you can delete content from your device manually.

* Upgrading to the latest version of iOS or iPadOS software provides the latest features, security updates, and bug fixes. Not all features are available on all devices or in all regions. Battery and system performance may be influenced by many factors including network conditions and individual use; actual results may vary.

This is from Apple's Support Page

iOS 12 is not available anymore, you have missed the boat so to say.

Which iPad model do you have?

See Settings > General > About > Model Number > tap here and tell us the number starting with A????

Anyway, you need to delete iOS 12 and start again with the latest iOS/iPadOS version compatible to your device.

Please read this article: If iTunes can't contact the software update server for iPhone, iPad, or iPod - Apple Support

Hope you find a solution!

It says to connect to iTunes.

1. On a Mac with macOS Catalina 10.15, open Finder. On a Mac with macOS Mojave 10.14 or earlier, or on a PC, open iTunes.
2. Connect your device to your computer.
3. Locate your device on your computer.
4. Click General or Settings, then click Check for Update. 
5. Click Download and Update.
6. If asked, enter your passcode. If you don't know your passcode, learn what to do.