My iPhone won’t update to the latest update. Why?

Also make sure, you're running the latest version of iTunes:

Update your iPhone, iPad, or iPod touch - Apple Support

If you are truly referring to your iPhone 8, the most common reasons for updates that refuse to start are battery at less than 50% charge level and no WiFi or iTunes connection. Sometime during the last year, Apple stopped permitting iOS updates via cellular data, so now you need to be connected to the internet by WiFi or computer running iTunes. Only they know why.