pgmkernel: 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/pgmkernel - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/pgmkernel - from package: netpbm

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

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