1. Get hardware address to which Serial-USB converter is attached
$ dmesg | grep tty
dmesg (display message or driver message) command is useful to gather hardware information loaded into kernel at the booting time or when a plug and play device is connected. You will get result something like below. Note down the port value, in my case it is ttyUSB0
usb 3-4: FTDI USB Serial Device converter now attached to ttyUSB0
2. Provide permission to this port
$ sudo chmod 666 /dev/ttyUSB0
You need to provide permission everytime you connect your serial-usb converter.
3. Try with Putty now to connect to your Serial device, it should work!