If you are getting trouble while unlocking by Nokia Lumia WP8.0 & WP8.1 Series Direct Unlock by USB Cable service, please follow the guidelines below. 

Download The NkWp8Client
There are different versions Wp8Client. so you should try a different version of the latest version of Wp8Client with unsuccessful.

*Make sure, your data cable is good or try to use another good quality data cable.
* Make sure your phone driver installed properly.
* Make sure your Desktop/Laptop Windows Firewall is Turn OFF or you may try to Turn On if Turn OFF (For making sure if any conflict with windows firewall).
* Make sure No Space before/after while entering the Unlock KEY.
* Make sure your phone is listed support models.
*Make Sure your ISP connection is directly connected to your Desktop/Laptop.

If above guidelines applied and still getting an error, please try to follow the additional guidelines below.
1st. Try to Hard Reset of your phone.
2nd. Try to flash your phone with a different version of firmware of the same model (We recommended, better also change product code).
3rd. Try to unlock by another Desktop/Laptop (We recommended, make sure another Desktop/Laptop installed with different version of windows operating system).

If above guidelines applied properly and finally unlock unsuccessfully, please click the Verify button from order history for further verification.

Thanks for your feedback!