The USB output appears in the output list only when a USB DAC is connected, powered on and recognized. If your USB DAC isn't detected or doesn't play, try the following steps.
Connect the DAC via USB and power it on before powering on your Volumio device.
Go to
Settings→Playback Options→Output Device, select your DAC and save.Restart your Volumio device from
Settings→System.If it's still not detected, try a different USB cable and a different USB port.
The error Failed to open ALSA device usually means the selected output doesn't match the device currently connected. Selecting the output again and saving, as in step 2, normally solves it.
If no USB device is detected on any port, send us a log and contact our support team.