MAC address need to be 12 length of string that from 0123456789 and ABCDEF , any value other than those will be invalid to Operation System.

Good Example: [00:50:56:C0:00:08]

Bad Example: [00:50:56:C0:00:0X]

You can find your own PC's MAC address by do

1. cmd

2. ipconfig /all

Also, from netcut netcard interface, you can also get your netcard adapter's mac address information.