xmlsec1: 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/xmlsec1 - from package: xmlsec1

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

On Debian possible default full paths are:

/usr/bin/xmlsec1 - from package: xmlsec1

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

Error in other languages:
xmlsec1: bevel nie gevind nie
xmlsec1: comandă negăsită
xmlsec1: comando não encontrado
xmlsec1: commande introuvable
xmlsec1: command not found
xmlsec1: käsku ei ole
xmlsec1: không tìm thấy lệnh
xmlsec1: komanda nerasta
xmlsec1: Kommando nicht gefunden.
xmlsec1: kommandot finns inte
xmlsec1: komut yok
xmlsec1: nie znaleziono polecenia
xmlsec1: níor aimsíodh an t-ordú
xmlsec1: no se encontró la orden
xmlsec1: no s'ha trobat l'ordre
xmlsec1: opdracht niet gevonden
xmlsec1: parancs nem található
xmlsec1: perintah tidak ditemukan
xmlsec1: príkaz nenájdený
xmlsec1: příkaz nenalezen
xmlsec1: команда не найдена
xmlsec1: командата не е открита
xmlsec1: 命令找不到
xmlsec1: コマンドが見つかりません