Looks like this phone does not support flashing partitions via the fastboot utility. According to this guide you will first need to root the phone by following the instructions from here:
Download the latest version of Framaroot and side-load the APK.
Open Framaroot select the action to perform. Select “Install SuperSU”.
In the rooting menu, Select an exploit in list above to potentially root your device as Barahir or BOROMIR and proceed
further. The rooting has now started and you will be getting an
awesome success message.
After the end of the installation process, reboot your device and you can now enjoy the Rooted Lenovo s920.
After the phone is rooted, you will need to do the following:
Try using SP Flash tools. It's for MediaTek devices like yours. You will need the correct drivers. Look this up on XDA, there is a fair amount of risk involved. You MAY BRICK THE PHONE. I have a Canvas Knight, and the same problem occurred with me too. This ASE post(Thanks to @Gulam Mohammed for his detailed answer) has further guides (although it is not excatly for your device, but it is relevant).