ppmimgvquant: 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/ppmimgvquant - from package: imgvtopgm

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

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