grub-reboot: 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/grub-reboot - from package: grub2-common
/usr/sbin/grub-reboot - from package: grub-legacy

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

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