haproxy: 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/haproxy - from package: haproxy

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

On Debian possible default full paths are:

/usr/sbin/haproxy - from package: haproxy

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

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