pgmmorphconv: 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/pgmmorphconv - from package: netpbm-progs

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

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