Last free update is not having the right files to play the game...
Viewing post in Stranded in Space - Free Edition - Days 12-13 Out Now! comments
Make sure you have 7Zip or Winrar, you have to unpack the update file then drag/drop all the files within into your "game" folder, overwriting what wants to be overwritten.
It's just the update files, so Days 1-11 (free version or otherwise) are needed. This was done to reduce file size downloads, as the full game is close to 8 gbs.