fence_apc_snmp: 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_apc_snmp - from package: fence-agents

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

On Debian possible default full paths are:

/usr/sbin/fence_apc_snmp - from package: fence-agents

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

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