ppm2mpeg: 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/ppm2mpeg - from package: gerris-mpi
/usr/bin/ppm2mpeg - from package: gerris

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

Error in other languages:
ppm2mpeg: bevel nie gevind nie
ppm2mpeg: comandă negăsită
ppm2mpeg: comando não encontrado
ppm2mpeg: commande introuvable
ppm2mpeg: command not found
ppm2mpeg: käsku ei ole
ppm2mpeg: không tìm thấy lệnh
ppm2mpeg: komanda nerasta
ppm2mpeg: Kommando nicht gefunden.
ppm2mpeg: kommandot finns inte
ppm2mpeg: komut yok
ppm2mpeg: nie znaleziono polecenia
ppm2mpeg: níor aimsíodh an t-ordú
ppm2mpeg: no se encontró la orden
ppm2mpeg: no s'ha trobat l'ordre
ppm2mpeg: opdracht niet gevonden
ppm2mpeg: parancs nem található
ppm2mpeg: perintah tidak ditemukan
ppm2mpeg: príkaz nenájdený
ppm2mpeg: příkaz nenalezen
ppm2mpeg: команда не найдена
ppm2mpeg: командата не е открита
ppm2mpeg: 命令找不到
ppm2mpeg: コマンドが見つかりません