zabbix_agent: 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/zabbix_agent - from package: zabbix20-agent
/usr/sbin/zabbix_agent - from package: zabbix22-agent
/usr/sbin/zabbix_agent - 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_agent - from package: zabbix-agent

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

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