Hi Leigh,
the error message is a bit misleading in this context, but:
to create the elevation diagram, the plugin needs to parse
the gpx file. If you set "read track infos" to "no",
it will not parse the file and thus you'll get no
elevation diagram (and as a consequence, it can not be written either).
Simply set "read track infos" to "yes" and all should be fine.
cheers, Frank