A few users have encountered similar, when the Narration is on the Narration Track. It is one of the two "fixed" Audio Tracks in the Timeline.
Some have had success by moving their Narration to another regular Audio Track, say Audio 3, if available.
One little check, before doing this is to open Audio Mixer, and watch the Levels in the Narration Track, during playback.
Even when there are no Track Keyframes (created with Audio Mixer), and no Clip Keyframes (created by several different methods), some still cannot get Export from the Narration Track, and for those few, I have not seen a real fix.
Thanks Hunt. I should have tried shifting the audio tracks around before - that's solved it. One of those quirks I suppose!
Great news! I only wish that I knew the variables that seem to shut down the Narration Track. Obviously, in previous cases, we've tried all of the "usual suspects, but there seems to be something else, that comes into play, and I've not heard of a "solution," or even a real "cause."
Glad that things are working for you, and thank you for reporting,