rpmbuild: 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/rpmbuild - from package: rpm-build

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

On Debian possible default full paths are:

/usr/bin/rpmbuild - from package: rpm

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

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