It's likely a server configuration and/or files permission problem.
If you have uploaded an index.htm or index.html or default.htm or whatever your server considers the "default" file for directories, and if the web server is configured not to allow default listings, then your files are probably not readable by the web server user account. In this case uploading the files and specifying "set permissions" to rw-r--r-- (+x) may fix the problem.
If the web server is configured to allow default directory listings, then the problem is that the directory from which your web page is being served is not readable by the web server user account. Correcting that will require changing the permissions on the directory that acts as / for your website, or changing the web server configuration.
Or it could be any number of other issues, but start with those. None of these are problems with WinSCP itself.
If you have uploaded an index.htm or index.html or default.htm or whatever your server considers the "default" file for directories, and if the web server is configured not to allow default listings, then your files are probably not readable by the web server user account. In this case uploading the files and specifying "set permissions" to rw-r--r-- (+x) may fix the problem.
If the web server is configured to allow default directory listings, then the problem is that the directory from which your web page is being served is not readable by the web server user account. Correcting that will require changing the permissions on the directory that acts as / for your website, or changing the web server configuration.
Or it could be any number of other issues, but start with those. None of these are problems with WinSCP itself.