No inserting a buss won't fix it, but busses are very useful. Reading and working through the tutorials is a good starting point if you are new to recording. It's a steep learning curve but there's plenty of great learning resources on the web, including here of course.
Have you checked all the output drivers in the audio preferences->audio->devices dialogue?
It sounds to me like you may have your output set to only one side of your output. Hopefully someone with the same interface will jump in and help but in the meantime I'll try and help as best I can.
I see the U-2000 is a 16 in 4 out interface, that means that in your audio devices list there should be 16 In drivers listed, probably reading something like......
Tascam 2000 Left 1/2, Tascam 2000 Right 1/2, Tascam Stereo 1/2 etc etc.
and 4 out in your audio outputs, something like.........
Tascam 2000 main Output 1/2 Left, Tascam 2000 main Output 1/2 Right, Tascam 2000 main Output 1/2 Stereo..... etc, etc.......
My wording could be way way off but you should see something similar. You need to make sure all these are checked so that they are available for use within Sonar.
If you start a default project and IIRC correctly Sonar will start you with a template that has some tracks in it and also a couple of busses, one called metronome and one called master.
Make sure you can see you "Buss pane" which in the track view is below the "tracks pane". Check in the buss pane that your "Master buss" output is set to one of your "Stereo" outs and not one side.
Read Tutorial one for more info on setting outputs etc.
That should at least get you going with sound from both sides.