grub-mkconfig: 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-mkconfig - from package: grub-common

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

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