pbmmask: 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/pbmmask - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/pbmmask - from package: netpbm

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

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