pnmmontage: 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/pnmmontage - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/pnmmontage - from package: netpbm

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

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