pgmenhance: 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/pgmenhance - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/pgmenhance - from package: netpbm

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

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