pgmabel: 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/pgmabel - from package: netpbm-progs

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

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