fence_pcmk: 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 CentOS possible default full paths are:

/usr/sbin/fence_pcmk - from package: pacemaker

You may install the required package with command yum install {package}

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