You're not alone. It seems as though Samsung has re-written the included Contacts app and removed this option. It's been brought up related to the S8 Oreo update also, so it seems like a new direction for Samsung.
Some texting apps allow you to set a per-thread notification sound. Once you have a thread started with a contact, see if you can adjust the thread options for that thread in your messaging app to set the sound.
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