Unfortunately, it seems that Google has removed the ability to assign contact-specific notification tones from Android 8.0 (Oreo), so it isn’t possible at the moment. Of course, it’s possible that they’ll add this feature back in at some point, but nobody here is able to predict when (or even if) that will happen.
One potential work-around exists, but it involves using Android Messages instead of the stock Samsung messaging app.
Download Android Messages from the Google Play store https://play.google.com/store/apps/details?id=com.google.android.apps.messaging, then open the app and start a conversation with someone. While in the conversation view, tap the menu button (three vertically-stacked dots in the upper right corner of the app), then tap “People & options.” This will present you with the option to assign a ringtone to this conversation. If you ever delete this conversation, however, you’ll have to go through this process again to re-assign your desired ringtone.
If you decide to use Messages as a work-around, you’ll also want to make it your default messaging app. On your Galaxy, open Settings -> Apps, then tap the three-dots menu in the upper right. Tap Default Apps -> Messaging app, and change the default to Android Messages.