du: 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/du - from package: coreutils

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

On Debian possible default full paths are:

/usr/bin/du - from package: coreutils
/usr/lib/plan9/bin/du - from package: 9base

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

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