Help me understand why Black can't use the King capture the Queen here?

Because the knight covers the queen, and you cannot move into check.

But that knight is pinned by the rook on d8, right?

Which king is in check?

The Black king would be taking the white Queen, which isn't really covered by the knight on d5 since that knight is pinned. So I assume the Black king is free to take the white queen but the move isn't being allowed

Youre in check.  You cant take the queen because you would still be in check.

This topic comes up from time to time. A pinned piece doesn’t lose its ability to protect a piece from the opposing king.
Think of it this way: if b4 was an empty square, would the Black K be allowed to move there? Of course not, because that would be moving into check. Same thing with the Q on that square. It’s that simple.

OK thanks, I thought since the knight can't move at all currently that space would be open to move to. Thanks everyone, pretty new to chess here.

Yeah, it's a common misunderstanding based on the idea that a pinned piece means it can't move.  That's not exactly the right way to think of it.  A piece being pinned means that if you move it, then you expose a piece to attack.  If that piece is a king, then it is an illegal move.  A pinned piece not being able to move is, thus, a consequence of the rule that you are not allowed to expose a king to attack, not a rule in and of itself.

If you remove that rule (and say that capturing a king would end the game), then you look at who loses the king first.  After Kxb4 (exposing the black king to attack by the knight), Nxb4 (exposing the white king to attack by the rook) ends the game, and black doesn't have time to capture the king on d1.

     Should you be allowed to take the white queen with your knight, you would lose as the white knight would capture your king before you could capture theirs. with your rook. Of course, both moves are illegal but that is the logic behind it.

     The same principle confuses some beginners when it comes to castling. It is illegal to move your king onto or through a square that is covered by an opposing piece, even if the piece is pinned to their own king. Same underlying logic--they kill your king before you can kill theirs.

     Remember that chess was originally based on warfare in an era when a king was considered Heaven's choice to rule, perhaps even own, the whole nation and all its inhabitants. Many ancient and medieval battles were  decided by the death or capture of the king, even if his army was routing the enemy.

"In this position, black took the queen on b4, offering an exchange of kings"

Ok thanks! I totally get it now and it makes a lot more sense. 

Chess is played in turns, right?

When you capture the queen with your king, knight will capture your king in the next turn and game will be over.

Your rook won't have any chance to attack enemy king because game was over one turn ago.

Of course kings can't be captured but i tried to tell it simple so you could understand better. Very simply; "Kings can never move into checks" memorize it as a rule.

Hope it helps.

Notice the contradiction. Your argument is essentially "black should be able to put his king in check, but white is not allowed to."

The reason exposing your King to "check" is considered an Illegal Move is literally to prevent players from blundering their King, since doing so loses instantly.  Saying, "That's an illegal move, since you're in check" is kinda like saying, "If you do that, I'll just take your King, and then I win, so take that back and try another move."  Looking at it that way, the position in the OP is more clear, I hope.

but the white knight will kill the black first then game over so the king cant capture the queen

I think these "who would capture the other King first if it could move" thing is not the point here. The real point is: The King is never be able to go to a field where he would be in check! And that a piece is not able to move (pin!) does never mean its ability to "cover" some fields by its basic movement ability is affected. The King here would be in check if he would take the Queen in b4, and that is absolutely not allowed.

it's alright it does sound weird

As many others put this

The game prematurely ends when a king is in danger and cannot escape. If we where to extend that to a logical extent of, whoever loses their king loses the game, you lose your king first, and the game is over. 

Lmao