@Nescio @Panther
Thanks for your replies, I figured it out why the errors occurred. It's because I created many civ.xmls but it was empty inside.
The main problem now is that "StartEntities": [] seems to be not working. When I start the game using my civ. It said that it cannot load the unit file, but the unit file exists. Also removing the units in StartEntities, only leaving civil_centre not work as well.