I enabled jailshell for my clients but cpanel jailshell seems to have some problem as one of my client can't even run the command 'dir'.
Can someone advise?
Maybe because 'dir' is a dos command, not a unix one. Try 'ls' :-P
Jokes aside, assuming /etc/passwd is correctly pointing those user shell to "/usr/local/cpanel/bin/jailshell", i would check integrity of binaries: running that shell from root and troubleshooting what could be wrong.