It's does look like your USB dac is not been recognized. Turn on the DAC and set it to USB. Then go to Main Page and hit restart. Then go back to output setting and select HW:CARD.
I see that you are using PP with a Rpby Pi4. I was planning to retry it with a Pi3, as I had some issues with LMS that I think have been solved. Also working on assembling my media server with a Linux PC, using Truenas as NAS. The media server will be both for audio & video.
The Rpbi3 USB output will be connected to a Topping D50. When I first assembled it, the audio output was the best I had ever heard here.
For the video output I'm using an Nvidia Stealth and Kodi. Kodi is not that good for audio as the other setup.
The Topping D50 arrangement should be the same as for the SMS.