pbmtopk: 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/pbmtopk - from package: netpbm-progs

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

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