Your navmap part of NCX code may like this
It's a blank NCX file without TOC, such epub can be read properly
on ADE and iPad, but can't pass epubcheck, which will report
element "navMap" incomplete; missing required element "navPoint"
first playOrder value is not 1
ePub Maker before 1.2 also generates such code if there's no TOC in Word,
also received such error code from epubcheck.
I guess Smashword also has such a problem.
You may have to insert a navpoint manually to suppress this error, or you may use another tool to convert.
By the way, the error tip from epubcheck is generally weird, misleading or meaningless.