grub-mkpasswd-pbkdf2: 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-mkpasswd-pbkdf2 - from package: grub-common

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

Error in other languages:
grub-mkpasswd-pbkdf2: bevel nie gevind nie
grub-mkpasswd-pbkdf2: comandă negăsită
grub-mkpasswd-pbkdf2: comando não encontrado
grub-mkpasswd-pbkdf2: commande introuvable
grub-mkpasswd-pbkdf2: command not found
grub-mkpasswd-pbkdf2: käsku ei ole
grub-mkpasswd-pbkdf2: không tìm thấy lệnh
grub-mkpasswd-pbkdf2: komanda nerasta
grub-mkpasswd-pbkdf2: Kommando nicht gefunden.
grub-mkpasswd-pbkdf2: kommandot finns inte
grub-mkpasswd-pbkdf2: komut yok
grub-mkpasswd-pbkdf2: nie znaleziono polecenia
grub-mkpasswd-pbkdf2: níor aimsíodh an t-ordú
grub-mkpasswd-pbkdf2: no se encontró la orden
grub-mkpasswd-pbkdf2: no s'ha trobat l'ordre
grub-mkpasswd-pbkdf2: opdracht niet gevonden
grub-mkpasswd-pbkdf2: parancs nem található
grub-mkpasswd-pbkdf2: perintah tidak ditemukan
grub-mkpasswd-pbkdf2: príkaz nenájdený
grub-mkpasswd-pbkdf2: příkaz nenalezen
grub-mkpasswd-pbkdf2: команда не найдена
grub-mkpasswd-pbkdf2: командата не е открита
grub-mkpasswd-pbkdf2: 命令找不到
grub-mkpasswd-pbkdf2: コマンドが見つかりません