key.dns_resolver: 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:

/sbin/key.dns_resolver - from package: keyutils

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

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