a2ping: 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/bin/a2ping - from package: texlive-utils

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

On Debian possible default full paths are:

/usr/bin/a2ping - from package: texlive-extra-utils

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

Error in other languages:
a2ping: bevel nie gevind nie
a2ping: comandă negăsită
a2ping: comando não encontrado
a2ping: commande introuvable
a2ping: command not found
a2ping: käsku ei ole
a2ping: không tìm thấy lệnh
a2ping: komanda nerasta
a2ping: Kommando nicht gefunden.
a2ping: kommandot finns inte
a2ping: komut yok
a2ping: nie znaleziono polecenia
a2ping: níor aimsíodh an t-ordú
a2ping: no se encontró la orden
a2ping: no s'ha trobat l'ordre
a2ping: opdracht niet gevonden
a2ping: parancs nem található
a2ping: perintah tidak ditemukan
a2ping: príkaz nenájdený
a2ping: příkaz nenalezen
a2ping: команда не найдена
a2ping: командата не е открита
a2ping: 命令找不到
a2ping: コマンドが見つかりません