grub-mkrelpath: 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/bin/grub-mkrelpath - from package: grub-common

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

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