ppmrelief: 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/ppmrelief - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmrelief - from package: netpbm

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

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