I encountered the same problem after a failed system update. Only way to fix was reloading the firmware over USB cable using ADB and FASTBOOT.
You'll need this file: https://www.leakite.com/2017/05/lenovo-t...
You need to install ADB and Fastboot for Android: https://www.androidpit.com/how-to-instal...
Once you have adb and fastboot installed, unzip the firmware file to the same folder as fastboot.exe
Also install Lenovo USB driver. You can download, or copy from the tablet itself using the USB cable.
Then power on the tablet holding Power button and Volume +. Release Power when lenovo logo pops up but keep holding Volume + until Recovery Mode comes up.
now select "reboot to boot loader" using the volume keys and press power to select.
now plug in USB cable and check that fastboot is detecting the tablet by typing "fastboot devices" to see if anything is found.
Now just run the flash-all.bat shell command from the fastboot directory and wait for it to complete.
The firmware will be reloaded but it will now be rooted. In order to do any system updates, you will need to select the full unroot option from the SuperSU app which will delete itself and reboot the tablet. There is also an app called FlashFire which you can uninstall as it no longer works anyway.
Once you do all this, everything should be back to factory condition. Now do the system updates one by one and make sure the tablet is fully charged and the updates do not get interupted.