What is causing the error in Codeigniter 4?

Firstly, have you checked to make sure that the "booking.php" file is located in the correct directory and has the correct file name? Additionally, have you checked to make sure that the file permissions for the "booking.php" file are set correctly?

Another thing to check is the routing configuration in CodeIgniter. Make sure that you have defined a route for "/booking" in the "app/Config/Routes.php" file. You can define a route like this:

$route['booking'] = 'Home/booking';

If none of these solutions work, you may want to consider checking the error logs to see if there are any specific error messages that can help diagnose the issue.