Note: Xperia z2 tablet is not "officially" supported on flashtool. This is be known to have errors when trying to unlock the bootloader and getting the message that it still needs to be in fastboot mode when it is clearly in fastboot already. It does however read the devices status and connect thru adb. WiFi 16 GB model has provided numerous issues in my experience especially.
Want to unlock bootloader and throw a custom recovery on it? As of now, voiding warranty thru Sony for the unlock code and pushing it thru command mode fastboot is your cleanest solution.
"Educate through persistent research"