ppmmake: 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/ppmmake - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmmake - from package: netpbm

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

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