disk-manager: 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/sbin/disk-manager - from package: disk-manager

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

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