Parallels Desktop displays the following error:
Unable to save the configuration because it is invalid.
This may happen if the MAC address in the virtual machine configuration is incorrect.
1. Make sure your virtual machine is stopped;
2. Open your virtual machine configuration editor, go to Hardware tab and click on your network adapter (check all of them if necessary):
3. Make sure that MAC address there complies with the following format:
XX:XX:XX:XX:XX:XX, where X is either a number from 0 to 9, or a letter from A to F, case-insensitive.
See this Wikipedia article for more details:
4. If you are not sure or there is no difference which adapter you have, you may press the Generate button on the right. It will generate a valid MAC address for your virtual machine.