pnm2ppa: 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/pnm2ppa - from package: pnm2ppa

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

On Debian possible default full paths are:

/usr/bin/pnm2ppa - from package: printer-driver-pnm2ppa

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

Error in other languages:
pnm2ppa: bevel nie gevind nie
pnm2ppa: comandă negăsită
pnm2ppa: comando não encontrado
pnm2ppa: commande introuvable
pnm2ppa: command not found
pnm2ppa: käsku ei ole
pnm2ppa: không tìm thấy lệnh
pnm2ppa: komanda nerasta
pnm2ppa: Kommando nicht gefunden.
pnm2ppa: kommandot finns inte
pnm2ppa: komut yok
pnm2ppa: nie znaleziono polecenia
pnm2ppa: níor aimsíodh an t-ordú
pnm2ppa: no se encontró la orden
pnm2ppa: no s'ha trobat l'ordre
pnm2ppa: opdracht niet gevonden
pnm2ppa: parancs nem található
pnm2ppa: perintah tidak ditemukan
pnm2ppa: príkaz nenájdený
pnm2ppa: příkaz nenalezen
pnm2ppa: команда не найдена
pnm2ppa: командата не е открита
pnm2ppa: 命令找不到
pnm2ppa: コマンドが見つかりません