Google's ARCore SDK currently doesn't support Samsung's latest flagship smartphones - the Galaxy S9 and S9+. However, that's going to change soon. As per a member of Google AR team, support for these devices is coming soon.
Here's exactly what they said:
Support for the S9 and S9+ will be added in the next release, which should be released in the coming weeks.
In case you aren't aware, ARCore is Google's answer to Apple's ARKit. It's aimed at enabling augmented reality experiences for existing and upcoming Android smartphones without the need for any additional hardware.