sex: 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/sex - from package: sextractor

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

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