parallel: 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/parallel - from package: moreutils

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

On Debian possible default full paths are:

/usr/bin/parallel - from package: moreutils
/usr/bin/parallel - from package: parallel

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

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