The MS Wavetable synth cannot be bounced. It is essentially an external device. To bounce your MIDI to audio, you must have a soft synth inserted in your project. It would have a synth audio track, in addition to the MIDI track you are seeing.
Considering the vintage of your software, the Edirol VSC synth is a likely option. Like the MS Wavetable synth, it is a GM synth, but it operates within the framework of Cakewalk/Sonar.
Insert that, direct your MIDI track output to it, and it will provide the missing audio that the bounce function is looking for.