ppmquant: 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 CentOS possible default full paths are:

/usr/bin/ppmquant - from package: netpbm-progs

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/ppmquant - from package: netpbm

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

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