pnminterp: 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/pnminterp - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/pnminterp - from package: netpbm

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

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