I couldn't convert to KFX because it was missing a file - a gif, which doesn't make any sense for a book - but I could convert to azw3. Do you know why?
calibre, version 4.5.0
ERRO: KFX conversion failed: <p><b>Cannot convert How to Fail at Almost Everything and Still Win Big</b><br><br><b>Conversion error:</b> Kindle Previewer error: Error:E00001: Cannot find the file name C:\Users\adria\AppData\Local\Temp\calibre_nw0p2q\k c_hqh\eqlf2lcxklyjin3uni4w\0000\conv_resources\con v_tmp\temp\extractedEpub\OEBPS\styles\images\00004 .gif</p>