flexpart.ecmwf: 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/flexpart.ecmwf - from package: flexpart

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