iPad won’t update the iOS.

If you have a computer, use that by connecting the iPad to the computer. You are jumping 3 iOS versions which is a lot for an OTA update. If the Update option does not work on the computer, then use the Restore option which will wipe the device and let you restore from a backup.

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. Update via Cellular connection, where available, requires a high-speed 5G connection.

You will only be able to update your iPad to the most recent version of iPadOS supported by your iPad - at the time of writing, this being iPadOS 26.2.

iPadOS 18 is no longer being digitally signed, by Apple, for your iPad.

David M Brewer wrote:

I don’t have a computer. The update is iOS 18 or go directly to 26. I haven’t tried iOS 26. Like I said I tried everything but iOS 26. After reading everything the conclusion is to wipe the drive and install either 18 or 26. Can I just wipe it clean and set it up as a new iPad. Then install from my back up?

Gotcha, I saw 16.4 in your original post. Yes, I would wipe and restore from backup which would take you to iPadOS 26.2.

David M Brewer wrote:

I don’t have a computer. The update is iOS 18 or go directly to 26. I haven’t tried iOS 26. Like I said I tried everything but iOS 26. After reading everything the conclusion is to wipe the drive and install either 18 or 26. Can I just wipe it clean and set it up as a new iPad. Then install from my back up?

LotusPilot is correct. iOS 18 isn't an option anymore, so if you've been trying to avoid iOS 26, that would explain why you can't update.