ff-c++: 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 Debian possible default full paths are:

/usr/bin/ff-c++ - from package: freefem++

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

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