ppmlabel: 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/ppmlabel - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmlabel - from package: netpbm

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

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