iPad update will not install

You may need to update your iTunes version first, then download the latest version for the iPad that is available. Normally iTunes will tell you if there is an update available. If you’re trying to restore then you need to first update the iPad to a later version then apply the restore from iTunes... see if this helps... https://support.apple.com/en-gb/HT204204

I am assuming you are on ipad 2? Try to look in your storage space in settings-general storage and icloud usage - mange storage

There maybe an older ipad software partially downloaded.

If you still do not see it there it would plug into a computer with iTunes and download it there

The current iOS level is 11.4.1. I have no idea where the one you referenced is coming from (unless that is the level your model of iPad will accept).

Barry

Also, make sure you’re connected to your iPad using a verified genuine usb cable as it often errors of you use a third party cable when trying to sync or download over iTunes to iPad