Both Galaxy S9 and S9+ are now present in the official ARCore supported devices list. Plus, users are also reporting that ARCore works on their units.
To use the feature, first you need to download the ARCore app from the Play Store. Then, install any app that utilizes the SDK (you'll find some examples in the Reddit discussion linked at the bottom).
If things don't work this way, you may try rebooting and re-installing the apps, as this has worked for some.
To quickly refresh, ARCore is Google's answer to Apple's ARKit. The solution is aimed at enabling augmented reality experiences for existing and upcoming Android smartphones without the need for any additional hardware.