y4mtoppm: 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/y4mtoppm - from package: mjpegtools

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

Error in other languages:
y4mtoppm: bevel nie gevind nie
y4mtoppm: comandă negăsită
y4mtoppm: comando não encontrado
y4mtoppm: commande introuvable
y4mtoppm: command not found
y4mtoppm: käsku ei ole
y4mtoppm: không tìm thấy lệnh
y4mtoppm: komanda nerasta
y4mtoppm: Kommando nicht gefunden.
y4mtoppm: kommandot finns inte
y4mtoppm: komut yok
y4mtoppm: nie znaleziono polecenia
y4mtoppm: níor aimsíodh an t-ordú
y4mtoppm: no se encontró la orden
y4mtoppm: no s'ha trobat l'ordre
y4mtoppm: opdracht niet gevonden
y4mtoppm: parancs nem található
y4mtoppm: perintah tidak ditemukan
y4mtoppm: príkaz nenájdený
y4mtoppm: příkaz nenalezen
y4mtoppm: команда не найдена
y4mtoppm: командата не е открита
y4mtoppm: 命令找不到
y4mtoppm: コマンドが見つかりません