xml2-config: 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/xml2-config - from package: libxml2-devel

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

On Debian possible default full paths are:

/usr/bin/xml2-config - from package: libxml2-dev

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

Error in other languages:
xml2-config: bevel nie gevind nie
xml2-config: comandă negăsită
xml2-config: comando não encontrado
xml2-config: commande introuvable
xml2-config: command not found
xml2-config: käsku ei ole
xml2-config: không tìm thấy lệnh
xml2-config: komanda nerasta
xml2-config: Kommando nicht gefunden.
xml2-config: kommandot finns inte
xml2-config: komut yok
xml2-config: nie znaleziono polecenia
xml2-config: níor aimsíodh an t-ordú
xml2-config: no se encontró la orden
xml2-config: no s'ha trobat l'ordre
xml2-config: opdracht niet gevonden
xml2-config: parancs nem található
xml2-config: perintah tidak ditemukan
xml2-config: príkaz nenájdený
xml2-config: příkaz nenalezen
xml2-config: команда не найдена
xml2-config: командата не е открита
xml2-config: 命令找不到
xml2-config: コマンドが見つかりません