zabbix_agentd: 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_agentd - from package: zabbix20-agent
/usr/sbin/zabbix_agentd - from package: zabbix22-agent
/usr/sbin/zabbix_agentd - from package: zabbix-agent

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

On Debian possible default full paths are:

/usr/sbin/zabbix_agentd - from package: zabbix-agent

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

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