nbdev_export is to export contents of a folder that is initialized as a nbdev project (with nbdev_new as @chrwittm suggests) and expects certain settings if not specified differently. If you just want to export a single notebook you can use:
import nbdev
nbdev.export.nb_export('notebook-name.ipynb', 'path/where/to/save')
which @ThomasAlibert also described above 
Edit: nbdev_export to export a (nbdev) project; nb_export to export a (n)ote(b)ook