flex: 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/flex - from package: flex

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

On Debian possible default full paths are:

/usr/bin/flex - from package: flex-old
/usr/bin/flex - from package: flex

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

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