I can not make booking because booking request is sending for unlimited time and status bar show Ajax sending error. (Error: 500 Internal Server Error)?

Firstly you need to check access to the wpdev-booking.php file. If booking plugin is installed to the http://www.myserver.com/wp-content/booking/wpdev-booking.php so copy that link to your browser and check access. Its have to show this error: Fatal error: Call to undefined function get_option(). If you see something like:Error 500 Internal Server Error, so then you need to set access to the wpdev-booking.php file. In most case you just need to set atribute to that file as 755 using your FTP client, sometimes is needed editing .htaccess file. So contact your administrator or check .htaccess file by yourself.

Sometimes wp-content and/or wp-content/plugins and/or wp-content/plugins/booking directories and/or file wp-content/plugins/booking/wpdev-booking.php had permissions set to 777. So its also can make this issue. Please change permissions on those directories and file to 755 and it have to work.