The export function in SONAR runs under the assumption that you already have an audio track recorded. It sets it length based on the longest, or last-ending, audio file in the session. Make sure that you have an audio file (any one will do, as you can always delete it later) in your session, and that it extends to at least the length of your project. The audio file that you use will be replaced by an audio file of the synth mixdown.