I need to update my iPad but don't have a computer

You can use the over the air via WiFi Software Update option in the Settings app.

As I clearly stated.

Settings app, under General, on the right, tap Software Update, gives you option to install a software upgrade/update over your own home/local Wifi network.

https://support.apple.com/en-us/HT204204

Try this.

Make sure your Wifi connection is active and working.

Go into the Settings App, General Settings panel.

In the right hand column, tap iPad Storage

A list of every app will generate itself.

Once this list stops generating, look through this list until you see something like iOS 11.x.x in this list.

Tap on the gray disclosure arrow at the far right on that iOS 10/11.x.x panel.

It will bring you to another screen that will allowing you to completely delete/remove the entire iOS upgrade/update.

Then, do a hard reset of your iPad by holding down both the Home and sleep/wake buttons simultaneously until your iPad goes to black and restarts with Apple logo, then release the buttons.

Once your iPad resets and starts again, make sure your iPad is connected over your WiFi network.

Go Back to the Settings app, under the General category, on the right hand side, tap the Software Update and see if the Software Update appears, again, then tap on the update once again to see if the update will download and install to your iPad.

If successful and the recent update installs, once the installation is complete, do one more hard reset of your iPad by holding down both the Home and sleep/wake buttons simultaneously until your iPad goes to black and restarts with Apple logo, then release the buttons.

Good Luck!

Under Software update, could you tell me the what software update number the iPad wants to update to and the exact message you are receiving when you tap on Software Update function.

The Software Update version should be 11.4.