I was able to set a notification tone in safe mode. That is one piece of the puzzle. You don't say what to do next? Leaving safe mode just puts me back in the same place. Ther must be an aftermarket app that is blocking notifications. How do I find that short of doing a factory reset, and re-installing apps one at a time until I find the offending app. Just fix the problem!, and while you're at it fix the focus on the camera... it's horrible.
What I did to fix this was go to settings>>>>notification sound then I clicked the side button to turn the volume up.
Turns out if you pull the volume control down from the top when you hit the side volume button it will give you options for Notifications, ring tones, media etc. Hope this fixes it for you as well. Certainly annoying.
I'm having same issue. I went to the text conversation I was having with the wife. Pushed the 3 dots in top right and noticed that the notification sound was set to custom. I changed it to default and now it works. But you have to do it for EVERY conversation/contact. No way I'm doing that for every contact. Just gonna live with it until the bug is fixed.
I solved the problem, finally, by installing Google Messages and making it the default. I would guess that any Messaging app would work just as well. Something to do with the Samsung Messenger. I've been working on this problem for months and tried every solution suggested including settings reset and finally factory reset. This is the only solution that has reliably solved the issue of no notification sound when receiving text messages.
This also only lasted a little while.
My new solution that has lasted 2 days is Delete T-Mobile Visual Voicemail. Once I did that, sound notification has worked ever since.