fslex: 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/fslex - from package: fsharp

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