This means either another program is trying to use the COM you're trying to upload on, or the current program crashed previously trying to upload to the COM port, and it is now stuck.
Try the following:
- Unplug and re-plug in the device (this makes sure the last program disconnects from the device, as it is no longer there).
- Close any programs that might use the COM port. This includes SparkiDuino, PoMoCo, and any Arduino instances.
- Reboot the computer.