Solution 1The quickest solution for this issue is to contact your hosting provider and let them know the folder path and URL you are getting the 403, theywill be able to quickly update the rules on their servers.

Solution 2If you would like to try and fix the issue yourself there are many different techniques you can try, but remember every host is setup differently sosome of these options may not properly work on your hosts and others will.

1. How to Fix the 403 Forbidden Error in WordPress
2. Apache giving 403 forbidden errors – Stack Overflow
3. How to resolve 403 Access Denied message?

Solution 1Contact Host: This error indicates that your web server is not setup to serve up web pages. If your not familiar with setting up web servers such as Apache, then thequickest way to fix this issue is to work with your hosting provider and tell them you get a 404 at “XYZ” URL. They should be able to get you up and running in minutes.This is not something the plugin will be able to handle or fix as its a web server configuration issue.

Solution 2Apache Setup: When setting up the web server (typically Apache) one of the common overlooked setup parameters is the<Directory> directive. The following configuration will work on most setups but check withyour server admin to be sure on their server:<Directory />
Options Indexes FollowSymLinks Includes ExecCGI
AllowOverride All
Require all granted
Allow from all
</Directory>Solution 3Troubleshoot: To troubleshoot this issue place two files called test.html and test.php in the same directory as the installer.php file. Then try to browse out to both of them via a web browser. If neither work then your site is not setup to server from the location at which they are placed. If only the test.html file works then PHP may not be properly enabled on your web server. If your not sure about how to configure your web server to server from the proper path or serve up PHP files then contact your hosting admin as these are not issues the plugin will be able to fix.

Option 1Contact Host: The 405 Method Not Allowed is an HTTP response status code indicating that the specified request HTTP method was received and recognized by the server but the serverhas rejected that particular method for the requested resource. The bulk of 405 responses have to do with how the web server is configured.

The fastest way to resolve this issue is to work with the hosting companies technical support team. Typically what needs to be adjusted is how the web server excepts requests. The server error log on your hosts platform should help the system admin locate the specific request and adjust the server accordingly.

Option 2 Troubleshoot Server If you are familiar with troubleshooting web server issues you might have a look at this detailed 405 troubleshooting guide.

Options

1. Web Server: Check to make sure the web server is setup properly. Create a new file named test.php in the same location as the installer.php file. Inside the test.php file place the following code:
   

   <?php phpinfo(); ?>

   If this works and you see the text “PHP Version” at the top followed by other PHP settings info then continue to the other options below, if not then something is wrong with the web-server setup and you will need to contact your host or server administrator.

2. Server Logs: Examine the server error log as explained here. Also you may access all server error logs by going to cPanel > Statistics > Error Logs or using your host-specific control panel. If you don’t have direct access to the server logs ask your host support to look for any errors in the logs for you.
3. Permissions: Check the permissions on installer.php file and its parent directory. On most hosts it should be 755 and files should be 644. Be sure to double check with your hosting provider or server admin as to how they want permissions setup on their servers.
4. Change Browsers: Open another web browser and check for the white screen. If it works in the new browser then it may be a browser cache issue. You can usually fix this by holding down ctrl+F5 or manually deleting your browser cache.
5. Config Files: Backup and remove hidden configuration files such as .htaccess, web.config and .user.ini files (Apache, Microsoft IIS, WordFence). These filesunder certain circumstances have have configurations that prevent the installer from running. Simply remove them from the same directory that the installer.php fileis in and try to again. If the problem persist try and backup and remove the files from your root web directory, in most cases public_html.
6. Installer.php Some hosts and security software will automatically block requests to files named “installer.php”. In this case try to rename the file to something else.Using the ‘installer.php’ file name on a public server is not recommended and users should consider using the hashed installer by going to Duplicator > Settings > Packages Tab> Installer settings > Set to [name]_[hash]_[date]_installer.php
7. Reverse Proxy: When running behind a reverse proxy, the value of the original hostname called is in HTTP_X_FORWARDED_HOST. To get around this setup add the followingto the top of the installer.php and dup-installer\main-installer.php as such:
   

   $_SERVER[‘HTTP_X_ORIGINAL_HOST’] = $_SERVER[‘HTTP_X_FORWARDED_HOST’];

Also See:
How to handle “Internal Server 500” messages?