French Defense: Winawer, Advance Variation, 4...Nc6

For me the french defense is really hard to get a grip on. I do not do well with it. 

I played this variation and the computer says 4...Nc6 is a mistake, leaving white with an advantage of about +1.0. But I checked the database and this move has actually been played 22 times by masters. Since it has not been played much I was wondering if anyone could explain to me how or why this move is bad for black? I continued a few lines with the computer and didn't really get to a point where it looked like there was a clear advantage for white. 

What's wrong with 4...Nc6? 

I can understand why you don't do well with the French, cause it seems you are not familiar with a main idea in the French. The center that you see with d4-e5 / d5-e6 is not a super happy center for black. White has a space advantage. You want to attack it. You can only do that with ...c5 and ...f6. You might want to look into middlegame strategies where you play against a pawn chain. 

When the center is completely blocked (neither side can legally move either central pawn), the play is going to be on the wings.

When this happens, you have to look at the direction in which the blocked pawns point.  Black's pawn chain points towards the Queenside, White's the Kingside.  So that is the side you should be attacking.

Think to yourself and ask - What does 4...Nc6 do?  Here's what it does:

1) It blocks the Black c-pawn, another item of Black's that can attack d4.

2) By leaving the pawn on c7, it blocks the Queen from swinging out to the Queenside.  If the pawn were on c5, the Queen could come to b6 or a5, attacking the Queenside.

3) What do you do about the Bishop on c8. It often plays a defensive role on d7, covering what will be a weak e6-pawn once you play f7-f6, but another strategy that some use is to play b6 and Ba6, looking to trade off the bad bishop.  In this case, the Knight would need to be on b8 to be able to recapture on a6.  So Black has lost that opportunity too.

So if you can't bring your pieces out on the Queenside, you can't attack d4, you have no space on the Kingside, and your Queen is blocked from coming out anywhere, what will Black do?  The answer is that 4...Nc6 is a terrible move for all the reasons described in this post.

Black has 2 good options:

4...c5, immediately attacking the center

4...Ne7, leaving yourself flexible, giving White fewer options, but also committing Black to one of two lines if White stays with the main line with 5.a3.  Without ...c5, the 5...Ba5 option is terrible.  After 5.a3, it is 5...Bxc3+ 6.bxc3 and then 6...b6 or transpose to the main line with 6...c5.  The line that isn't very good for White after 4...Ne7 that can be a little annoying after 4...c5 is 5.Bd2.

The other main opening that involves blocked pawn chains is the Classical King's Indian.  I suggest you look at games with the French Defense, Advance Variation (C02), the French Winawer with 4.e5 (C16-C19), and the Classical King's Indian, Mar Del Plata and Bayonet (E97-E99).

4...Nc6 blocks ...c5.
However French with ...Nc6 is playable as well
https://www.chessgames.com/perl/chessgame?gid=1044354 

Apples to Oranges comparison.

In the Winawer, where 3...Bb4 and 4.e5 have already been played, 4...Nc6 is a terrible move.

And do not go cherry-picking for a single game where Black wins after 1.e4 e6 2.d4 d5 3.Nc3 Bb4 4.e5 Nc6? because for every one of those where Black wins, there will be numerous White wins.

I wanted to ask something about the French in general, Mr. ThrillerFan:

1. Why do you always say if your opponent plays the Exchange French you're happy? it's a draw - draws are gross.

2. How do we develop the kingside as Black if the opponent plays the Advanced French? [at My level, they only play Exchange or Advanced - other things are 'too much' for them lol, that's why the French is so great, besides of the solidity and queenside expansion].

Would I be correct in thinking then, that the Nc6 is less of a tactical error, and more of a strategical one? Or is this some tactical sequence of moves that will follow in which black will lose material? If chess is 99% tactics then what is the "tactic" in this instance? Or is that quote just an exaggeration and is not really true about chess? 

Absolutely a strategic error!

And sometimes, strategic errors are worse than tactical errors (not sure about this specific case) as it could impact the rest of your army for the rest of the game rather than merely something like a loss of a pawn.