Running Windows 7 64bit Ultimate and attempting a MAC scan via WIFI and getting this issue

Looks to be an issue with Cain and Abel according to this thread

http://oxid.netsons.org/phpBB3/viewtopic.php?f=8&t=4222

So to fix it edit the registry as this

That’s pretty strange… you are running Cain with administrative privileges right? If so, you can try editing the value directly in the registry. It’s located under HKEY_CURRENT_USER/Software/Cain/Settings within your registry. ensure Spoofing is set to 0.