parallel-ssh: 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 Debian possible default full paths are:

/usr/bin/parallel-ssh - from package: pssh

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

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