pdfstamp: 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/pdfstamp - from package: pdf-tools

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

On Debian possible default full paths are:

/usr/bin/pdfstamp - from package: libtext-pdf-perl

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

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