zabbix_sender: 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/zabbix_sender - from package: zabbix20
/usr/bin/zabbix_sender - from package: zabbix22
/usr/bin/zabbix_sender - from package: zabbix-agent

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

On Debian possible default full paths are:

/usr/bin/zabbix_sender - from package: zabbix-agent

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

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