hi, after installing zsh on Ubuntu server 18.04 LTS, i cant log in to root, it says No such file or directory

That's normal in all ubuntu versions, they do not use the root account, but setup sudo rights for the normal (first) user you specified in the install.
It IS possible to enable root but you have to do it using sudo from the normal user account.

how do i solve i

In general, although I've never done this myself:
login as your normal user.
become root with "sudo -s" (you may have to enter your password again)
now make sure that there are entries for root: in /etc/passwd and /etc/shadow,
if not, create them (they should be the first line).
the entry in the passwd file should look likeand the directory /root MUST exist.
use "passwd root" to set a GOOD (hard to break) password for that root user.
exit the sudo session
now try su to become root. If it works, log out completely and login again AS root.

PS: this sets only LOCAL access as root, remote you will still have to enter as "your normal user" and either use su or sudo, this is a security measure to prevent password carckers ON your server.