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