Why cant I get the customize button on my ipad lock screen?

iPadOS 16.1 does not [yet] support lock screen customisation. Likewise, the active Astronomy and Weather Wallpapers are currently exclusive to iOS16.x for iPhone. Whether or not Apple plan to bring these features to iPadOS is currently unknown.

Customising Lock Screens is only available on iOS 16, iPadOS 16 does not have this feature.