The latest APSC camera from Sony, the A6400 (ILCE-6400) has a new raw file format which is not supported by Darktable versions currently* in the Ubuntu repos or in the stable PPAs.
The only way to get darktable to open the RAW files from the A6400is to build the latest (unstable) darktable from source.
The steps to do this on a vanilla install of Ubuntu are given in a text file below, also available through a Github Gist
* as of this writing, Sep 5. 2019