Sometimes a web site allows a person to connect to the remote publish directory "/" but they cannot actually use that directory to publish to (that is, they do no have 'write access' to that directory) Instead, there is another remote directory (supplied by the host) that needs to be used.
In this situation, if you try to publish to "/" you would see the error "550...Cannot create a file when that file already exists...".
For example in some GoDaddy cases, for a domain name of "some_domain_name" the correct starting remote publish directory is "/some_domain_name" not "/". Even though you connect to both, you cannot publish to the latter.
To prevent this, you should double check the FTP starting location (remote publish directory) that your host has given you for html files.
If you do not have the exact remote directory supplied by your hosting company, the simplest way to get the default location is to leave the remote publish directory field completely empty, then press the Publish button.
When you do that, you will see a message that says "The remote publish directory has not been set. Would you like to use your server's current working directory?" Press the 'Yes' button and the current directory will be used as your remote publish directory.
Note that sometimes the server's current working directory is a starting point but not the exact folder that you want to place your html files in. In that case you would leave the remote publish directory blank but press the 'Browse' button. Then say 'Yes to the "...Would you like to use your server's current ..." question and then browse to the spot where you want you files to be. Press 'OK' and that location will be placed in your remote publish directory field.
If this does not work for you, you need to contact you host service and tell them the exact FTP information you are using to publish (host name, user name and remote publish directory) and ask them why you cannot send to your html files there. They should be able to tell you quickly what is going on.