Probably the folder is there already.
With your ftp client, browse to wp-content > plugins.
Do you see it there? then delete it and try again.
You can also download the plugin folder from the WordPress plugin page to your computer and ftp it to the wp-content/plugins folder.
This way it won’t give any problems because, if you already have the folder there, it will just ask you to overwrite and you can choose yes.