su: /bin/ksh: No such file or directory
 

I can't login as root:
what it show is....

[sidh@localhost ~]$ su
Password:
su: /bin/ksh: No such file or directory
[sidh@localhost ~]$

how to set root shell as bash or sh ?

Using "su" the manual page shows several options. If you can "su" to root, then you could try the following:

But that's just one-off. You'll need to use "usermod" to switch the default login shell permanently for root.

However, how is it that you have the account defaulting to a shell that apparently is not on your system?

There's usually something in /etc/profile.d/ related to shells. All shells also run a few scripts at logon; for bash, it's ~/.bash_profile & ~/.bashrc from memory.