ksh93: 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/ksh93 - from package: ksh

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

On Debian possible default full paths are:

/bin/ksh93 - from package: ksh

You may install the required package with command apt-get install {package}

Error in other languages:
ksh93: bevel nie gevind nie
ksh93: comandă negăsită
ksh93: comando não encontrado
ksh93: commande introuvable
ksh93: command not found
ksh93: käsku ei ole
ksh93: không tìm thấy lệnh
ksh93: komanda nerasta
ksh93: Kommando nicht gefunden.
ksh93: kommandot finns inte
ksh93: komut yok
ksh93: nie znaleziono polecenia
ksh93: níor aimsíodh an t-ordú
ksh93: no se encontró la orden
ksh93: no s'ha trobat l'ordre
ksh93: opdracht niet gevonden
ksh93: parancs nem található
ksh93: perintah tidak ditemukan
ksh93: príkaz nenájdený
ksh93: příkaz nenalezen
ksh93: команда не найдена
ksh93: командата не е открита
ksh93: 命令找不到
ksh93: コマンドが見つかりません