ppmmix: 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/ppmmix - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmmix - from package: netpbm

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

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