If you already have an FTP server running and attempt to configure CuteFTP Server to listen on the same IP and port, you will receive an error message stating that you cannot start that site. This is by design: you are not allowed to have two services listening on the same IP and port.
Either stop the other FTP server or configure the servers to listen on different IP addresses or ports.
If you have Microsoft Internet Information Services on your computer, see Unable to create socket on port 21, for instructions resolving IP conflicts with CuteFTP Server.