pbm2ppa: 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/pbm2ppa - from package: pnm2ppa

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

Error in other languages:
pbm2ppa: bevel nie gevind nie
pbm2ppa: comandă negăsită
pbm2ppa: comando não encontrado
pbm2ppa: commande introuvable
pbm2ppa: command not found
pbm2ppa: käsku ei ole
pbm2ppa: không tìm thấy lệnh
pbm2ppa: komanda nerasta
pbm2ppa: Kommando nicht gefunden.
pbm2ppa: kommandot finns inte
pbm2ppa: komut yok
pbm2ppa: nie znaleziono polecenia
pbm2ppa: níor aimsíodh an t-ordú
pbm2ppa: no se encontró la orden
pbm2ppa: no s'ha trobat l'ordre
pbm2ppa: opdracht niet gevonden
pbm2ppa: parancs nem található
pbm2ppa: perintah tidak ditemukan
pbm2ppa: príkaz nenájdený
pbm2ppa: příkaz nenalezen
pbm2ppa: команда не найдена
pbm2ppa: командата не е открита
pbm2ppa: 命令找不到
pbm2ppa: コマンドが見つかりません