This error occurs when the server was unable to process the request because access to the file was denied. It occurs most often in three cases:
1) there is no index file in the directory (index.html, for example) and at the same time the list of files in this directory* or in the entire virtual server is not allowed to be viewed. You can try creating a file called index.html and see if the error disappears.
2) the “permissions” for the file are such that the Apache web-server could not read the file on the server disk. Try changing the permissions to 640, for example.
3) user tried to access a script (in the cgi-bin directory) which has incorrect permissions – no read+execute permission (r+x) for that particular script. Change the rights to 755.
(By default, viewing the list of files in the directory is forbidden. In order to allow the output of the file list, you need to create a .htaccess file, in which you must write the Options +Indexes directive as a separate line).