grub-mkimage: 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-mkimage - from package: grub-common

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

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