ppmquantall: 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/ppmquantall - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmquantall - from package: netpbm

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

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