This means that your web server is configured to not allow direct access to Kickstart's PHP file.
If you are using the Apache or Litespeed web server (as is the case for most hosts and self-hosting environments) please rename or delete the .htaccess file from your site. This file, like all files whose name starts with a dot, is hidden on all server Operating Systems bar Windows Server. You will have to explicitly tell your FTP / SFTP client software, or your hosting control panel's file manager, to show hidden files before you can see and rename/delete this file. Alternatively, you can upload an empty plain text file named .htaccess into your site. Please remember that if you are using Windows the Windows File Explorer HIDES file extensions by default; when you create a new text file and try to rename it to .htaccess it is in fact named .htaccess.txt which is a DIFFERENT file, therefore this method WON'T work. Your file MUST be named exactly .htaccess with the dot in front.
Also remember that .htaccess files cascade through filesystem directories. If you are restoring your site in a subdirectory of your main site, or a subdomain whose web root directory is a subdirectory of your main site then your main site's .htaccess file will interfere. The only way to address this issue is to NOT do that kind of restoration. This is a feature and limitation of the Apache and Litespeed web servers. It is not under our control, and we cannot help with it. Even if we do spend copious amounts of effort and time to give you a workaround, the restored site will not work correctly anyway! Always restore in a domain / subdomain whose web root is NOT inside another site's web root.
If you are using the Microsoft IIS web server you need to delete or rename your web.config file for similar reasons.
If you are using the NginX web server you will have to edit the configuration of your site. Unlike Apache, Litespeed, and Microsoft IIS the NginX web server does not have an automatically applied configuration file which can be placed inside your site's web root. In some cases, some servers are configured with something similar (e.g. parsing an nginx.conf file placed in the site's root) but this is non-standard and requires restarting NginX or reloading the configuration of the NginX server to apply any changes.
In all cases please note that web servers do have site configuration which may be applying restrictions. Commercial hosts use third party or custom tools in their servers' configuration which may indeed prevent access to Kickstart. If unsure, please ask your host why you can't access Kickstart. Do not ask us for support, as we do not know your server's configuration, nor are we your host. We can only point out the possibilities stated in this documentation page which was written exactly because it's faster for you to read this information right away than waiting for us to copy and paste it to a support ticket response.