Hello again mates,
Thanks to Re4son, I can use root on kow now.
Now I encountered another problem here:
I have updated and upgraded my kali, and installed kali-linux package with apt-get successfully.

root@LAPTOP-N5F8O1GR:/# apt-get install kali-linux
Reading package lists... Done
Building dependency tree
Reading state information... Done
kali-linux is already the newest version (2018.2.0).
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

But when I tried to install kali-linux-full, some weird problems of dependencies caught me:


root@LAPTOP-N5F8O1GR:/# apt-get install kali-linux-full
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
kali-linux-full : Depends: acccheck but it is not installable
Depends: irpas but it is not installable
Depends: maltego but it is not installable
Depends: maltego-teeth but it is not going to be installed
Depends: nikto but it is not installable
Depends: wce but it is not installable
Depends: windows-binaries but it is not installable
Depends: wpscan but it is not installable
E: Unable to correct problems, you have held broken packages.



I tried installing those depended package, but still failed. For example:
root@LAPTOP-N5F8O1GR:/# apt-get install wce
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package wce is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'wce' has no installation candidate


By the way, I can not even use wget in my kow:
root@LAPTOP-N5F8O1GR:/# wget https://kali.sh/xfce4.sh
bash: wget: command not found


OK OK I admit I'm a **** junior rookie on linux here...... Thank you for your patient help mates