Why this move is blunder?

I think it's because your bishop is very bad on b5, it was best to take the center with c4 then put the bishop on d3.

You shouldn't take the word "blunder" too literally here. It is not like you dropped a piece, or missed an opportunity to win one. The mistake is purely positional. This makes it quite hard to explain precisely why Bb5 is so much worse than c4. I think these points are at least part of it:
* Going Bb5 and Bxc6 loses the bishop pair. Bishops tend to be especially strong together, so you should usually avoid trading a bishop for a knight when you still have both bishops.
* c4 is a standard way to play against the d5-pawn, which is well worth remembering. The idea is to trade black's d-pawn, which is a central pawn, for your c-pawn (central pawns are usually worth a little more). This lets you use the c-file, for example with Rc1. Also, it is not so obvious how black should deal with the tension. If he takes on c4, you take back with the bishop and it will be hard for black to castle. If he plays e6, then cxd5 exd5, and now you have achieved two things: black can never play e5 anymore (which was an important idea for black), and the d-pawn is now quite weak. You can attack it further with Nc3 and Qb3.

EDIT: I just realised you probably played Bb5 to prevent e5. If this is true, you correctly identified one of black's most important ideas. But it is worth remembering that, whenever your opponent already has a pawn in the center, and he is using a lot of resources to get the other one in the center as well, the first pawn often becomes weak. Notice how black's knight on c6 prevents black from putting his pawn there, and similarly the pawn on f6 stops the knight from going there. This should be a sign to you that d5 should be the square for you to focus on.

Playing the london is probably a blunder straight away. Also I think this move is speeding up black's development, because of what was played in the game, namely Bd7.

Black are not well developped. Bg7 is just bad and all the kind side.. C4 in view of opening center is more precise. But i don't think this is a blunder.

Thanks to all. I probably got the idea! Love you guys. You're awesome! ❤️

@ianhong Sorry! The server went down suddenly, maybe for approximately 15 minutes. After the connection was back, I found the game resulted in a draw.

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