I had the same problem. It turns out it is device specific. I used a different Android device and it worked fine for the setup. Then I was able to use it with all of my devices.
Now, however, I have a new problem. The soundbar will work just fine with Alexa for a while, then it stops working. I get a soundbar is unreachable message. I've tried powering it off and back on and I have verified that I have the latest firmware and the soundbar is connected to the internet.
The capture doesn't seem like your phone was not connected to your WiFi or the target product selection was not a soundbar though.
Why don't you reset the soundbar and try it again? (Press and hold "+"&"-" at a same time for 5 secs)
When I failed my setup, I found that the settings of SmartThings did not show me option to select available all were grey.
After off and on my iPhone6, it showed me available(the green coloured option..) and everything went smoothly.
The screenshots show WiFi as connected, not sure what you mean by "the target product selection was not a soundbar" given that it explicitly asks if I want to add the model of soundbar I have when I open the app (so it's clearly detected it). Resetting the soundbar as mentioned before doesn't help at all.
I get a picture of a soundbar when it pops up with the "Would you like to connect?" message. My images in the first post are from the connection process past that point.