That's the same error i was getting.
Double check your cables, and if you are using plugs on headers, back them out a bit..
I am assuming a dmesg is showing something like this when you plug in your cable:
[13880.473791] pl2303 2-1:1.0: pl2303 converter detected
[13880.485429] usb 2-1: pl2303 converter now attached to ttyUSB0
[13880.485475] usbcore: registered new interface driver pl2303
[13880.485480] pl2303: Prolific PL2303 USB to serial adaptor driver
OK, I just tested it with minicom in Linux. If you are using a debian variant (Ubuntu, Mint, etc), just do this:
sudo apt-get install minicom
sudo -D /dev/ttyUSB0
(assuming ttyUSB0 is what you saw in your dmesg output)
Then add a jumper wire between the send/receive wires with your serial adapter. (on mine, that is white/green)
Make sure you're not doing anything with the power from the serial adapter. Just TX/RX
Whatever you type should echo back.
If you aren't sure, do a CTRL-a and E (that turns on local echo) and you should see whatever you typed twice.
That way, you know your usb/serial adapter is working. Also, with a multimeter, you probably want to double check your voltage on the power wires to make sure that is working also.
Beyond that, it's probably the connection (bad connection, bad soldering, etc).