update-drbl-client-kernel-from-server: 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/share/drbl/sbin/update-drbl-client-kernel-from-server - from package: drbl

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

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