Try running as Administrator & also set the random mac address first 2 characters as 02 just like the msg says.Pc name can always be changed by going to system settings.
I changed a computer from "A" to "B" . I don't restart the computer. When I use System.Environment.MachineName I get "A" and not "B" - which correct. But when I see " System Property page" , " Computer Name tab " I see computer name as "B".
How to get changed Computer Name without restarting the computer
Why does computer name change require a reboot in Windows? - Server Faul
It may seem like it but complete pc name change requires reboot.As for mac address change error,did you select/tick the "use 02 as first octlet of mac address" option(3rd option) & untick all other network connections except wifi in the list before clicking the change now button.