Unable to update iPad 8th generation to iOS 18.2

According to this Apple Support page, your iPad 8 is not capable of updating that high, unless you are talking about an iPad mini.

iPad models compatible with iPadOS 18 - Apple Support

You may find this posting helpful.

Software update on 8th generation ipad - Apple Community

How much free space is there on your iPad? It could be that there is not enough to download the update. Usually it will tell you this though.

I have a 7th gen and an able to. While this isn’t an exact guarantee it works on 8th gen iPads as well, it seems quite likely. Were you able to update to 18.1 alright?

Strange, that is certainly enough space.