Transfer Files Flow Variable Issue

I’m using the Transfer File node to move files via SSH Connector to Google Drive. I can use a wildcard or regular expression to designate files to move, but what I actually want is to pass a single file name to it so I’m not always reimporting the same files since the host isn’t cleaning up old files. The use variable option is greyed out and none of the flow variables appear to accept a path or file name, so it’s unclear how this node should be accepting input parameters. Why are the variable settings disabled?