ltsp-update-kernels: 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/ltsp-update-kernels - from package: ltsp-server

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

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