standalone_httpd: 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

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/sbin/standalone_httpd - from package: rt3

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