This is the best advice I have seen for this problem, I wish I had seen it last night! I spent 4 hours trying to get the PC to recognise my A6400. Install drivers, uninstall, reinstall. Change settings etc.
It turns out it was actually a setting on the Sony camera called "Ctrl w/ Smartphone" in "Menu -> Network1" that was checked and not allowing the correct PC control function to operate. I had to change this to "Off". Then in "Menu -> Setup4" I could change "USB Connection" to "PC Remote".
After that the Sony A6400 is found in both ASIAIR menu and the N.I.N.A equipment menus (with the ASCOM Sony drivers for NINA).