ff-mpirun: 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

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-mpirun - from package: freefem++

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