Unable to open audio playback device

This can happen for a couple reasons. First, make sure the sample rate on the interface matches the intended sample rate in the project. Some interfaces will switch automatically, some won't.
 
Also try WDM instead of ASIO to see if that makes a difference. WDM can aggregate interfaces, so if another audio is doing something else in the system, it won't interfere with the device you're trying to sue with Sonar. This will also let you know if Sonar is seeing more than one device.
 
BTW my confidence in this advice is only about 60% but a least check these before trying other options.