The problems of adding user & changing user-group in CentOS

Problem 1

usermod: group 'sudo' does not exist

[root@server] $ adduser user-new
[root@server] $ usermod -aG sudo user-new
usermod: group 'sudo' does not exist  

CentOS doesn't have 'sudo' user group by default, but we can change user into 'wheel' user group, 'wheel' user group also has sudo permissions.

So we can do this:

[root@server] usermod -aG wheel user-new

Problem 2

(current) UNIX password:
passwd: Authentication token manipulation error

[root@server]$ su user-new
[user-new@server]$ passwd
Changing password for user user-new.  
Changing password for user-new.  
(current) UNIX password:
passwd: Authentication token manipulation error  

Wtf... What's UNIX password??? I have no idea about that...

Finally I found the way, we can just change the password under root user:

[root@server]$ passwd user-new
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