check_mk_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/bin/check_mk_agent - from package: check-mk-agent

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

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