fence_na: 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/fence_na - from package: fence-agents

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

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