No, port 21 is just the default for FTP. The error is categorical: “Connection Refused” means that packets from your PC are making it to the device at 192.168.11.1, and the device is actively refusing them (it’s sending a response back saying that it won’t allow the connection).
Looking again at your first screenshot, I see you’ve entered a path with spaces in. I wonder if that could be a problem. Could you try removing spaces from the paths, both in the UI and on your disk?
Also, can you confirm that your drive is formatted as FAT32, as mentioned in this note: