Apple Watch

First, not all complications can be placed in all possible complication positions. And some complications will not work with some watch faces. I do not know if that is the case for you and Messages, but I figured I would at least mention it.

This explains how to change various aspects of your watch face, including complications

https://support.apple.com/en-us/HT205536

This list watch faces and the Apple complications that are supported for each watch face

https://support.apple.com/guide/watch/faces-and-features-apd6ce85daf4/watchos

You can also use the iPhone "Watch" app to change watch faces and the complications on each watch face.