mpssh: 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:

/usr/bin/mpssh - from package: mpssh

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

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