NCX is required for epub2.
BUT
there may not be any entries other than start.
Conversion takes the input and converts.
No identifiable TOC (coded links) in the source and there will probably not be any entries. Just because there is a page that says Contents on it, does not make it one to conversions (or the device).
In those cases, a Human (you

) will be needed. You can add the Calibre TOC tool to your toolbar. This edits only the TOC of EPUB or AZW3