xbuild: command not found

Have You tried accessing the binary with the full path?

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/xbuild - from package: mono-devel

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

On Debian possible default full paths are:

/usr/bin/xbuild - from package: mono-xbuild

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