The AF Illuminator function on the external flash unit is designed for A-mount cameras (SLT and DSLR). It is not compatible with E-mount cameras, even if an A-mount adapter with Phase Detection AF is used.
In this case, using an external light source to help achieve focus is recommended if possible. If you cannot change the lighting conditions try selecting a different Focus Area or use AF-Lock on a higher contrast area. If the entire scene is very dark and has low contrast, use manual focus.