ksh: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/bin/ksh - from package: ksh
/bin/ksh - from package: mksh

You may install the required package with command yum install {package}

Error in other languages:
ksh: bevel nie gevind nie
ksh: comandă negăsită
ksh: comando não encontrado
ksh: commande introuvable
ksh: command not found
ksh: käsku ei ole
ksh: không tìm thấy lệnh
ksh: komanda nerasta
ksh: Kommando nicht gefunden.
ksh: kommandot finns inte
ksh: komut yok
ksh: nie znaleziono polecenia
ksh: níor aimsíodh an t-ordú
ksh: no se encontró la orden
ksh: no s'ha trobat l'ordre
ksh: opdracht niet gevonden
ksh: parancs nem található
ksh: perintah tidak ditemukan
ksh: príkaz nenájdený
ksh: příkaz nenalezen
ksh: команда не найдена
ksh: командата не е открита
ksh: 命令找不到
ksh: コマンドが見つかりません