update-openssh-known-hosts: 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/update-openssh-known-hosts - from package: openssh-known-hosts

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

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