ff-pkg-download: 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-pkg-download - from package: freefem++

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

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