VA: CNU - Clinton +15

[Alcon]

How can 6% of likely voters vote for "none" ?

"none" isn't even an option in VA.

I guess in this case it means that 6% of voters will invalidate their ballot ...

...No.  You can skip a race on the ballot without invalidating your entire ballot; it's called an "undervote" for that race.  There are more races on there than the Presidential.  I really doubt 6% of people will skip the Presidential, but some will, probably more than in most past years.