mandos-keygen: command not found

Have You tried accessing the binary with the full path?

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/mandos-keygen - from package: mandos-client

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