flexc++: command not found

Have You tried accessing the binary with the full path?

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 Debian possible default full paths are:

/usr/bin/flexc++ - from package: flexc++

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