pnminvert: 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/pnminvert - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/pnminvert - from package: netpbm

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

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