xmlmerge: 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/xmlmerge - from package: gwenhywfar-devel

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

On Debian possible default full paths are:

/usr/bin/xmlmerge - from package: gwenhywfar-tools

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

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