ppmtoy4m: 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/ppmtoy4m - from package: mjpegtools

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

Error in other languages:
ppmtoy4m: bevel nie gevind nie
ppmtoy4m: comandă negăsită
ppmtoy4m: comando não encontrado
ppmtoy4m: commande introuvable
ppmtoy4m: command not found
ppmtoy4m: käsku ei ole
ppmtoy4m: không tìm thấy lệnh
ppmtoy4m: komanda nerasta
ppmtoy4m: Kommando nicht gefunden.
ppmtoy4m: kommandot finns inte
ppmtoy4m: komut yok
ppmtoy4m: nie znaleziono polecenia
ppmtoy4m: níor aimsíodh an t-ordú
ppmtoy4m: no se encontró la orden
ppmtoy4m: no s'ha trobat l'ordre
ppmtoy4m: opdracht niet gevonden
ppmtoy4m: parancs nem található
ppmtoy4m: perintah tidak ditemukan
ppmtoy4m: príkaz nenájdený
ppmtoy4m: příkaz nenalezen
ppmtoy4m: команда не найдена
ppmtoy4m: командата не е открита
ppmtoy4m: 命令找不到
ppmtoy4m: コマンドが見つかりません