Can someone explain this position to me?

Why is a6 considered a mistake? After that move, I played b4, which is also marked as a mistake, and I don't understand why. Let’s say instead of playing a6, Black had castled. If I then played b4, it wouldn't be a mistake.

So what exactly makes a6 a mistake, and why does it make b4 a bad move afterwards, even though b4 would have been fine if Black had castled instead? I'd appreciate if someone could help break this down. I need to understand this so i can improve

the engine doesn't like it due to the move d4- which makes sense i guess because black has wasted a good amount of time with playing h6/a6

Bruh

okay but like i wrote in the post, Let’s say instead of black playing a6, Black had castled. If I then played b4, it wouldn't be a mistake. why is that not a mistake now but it would be a mistake if black played a4 instead of castle

I can answer that for you. a6 leaves black king in the middle of the board. So d4 comes in much stronger and much more effective. But if black castled instead and you played b4, it is seen as an acceptable idea as d4 isn't the same impact as it would have been with lines opening up to the king while it's still stuck in the middle

Hope this helps yeahhh

Okay thank you, i understand it much better now!
So the take away from this position is that blacks king is still in the middle!
My following questions just so understand it deeply enough, what if black played another move that a6 or castle, for example d6, would that change anything? i mean the king is still in the middle and black is not threatening anything?

d6 changes everything. Your d4 break isn't as good anymore since after an initial exchange, the d6 pawn controls the e5 square, preventing you from immediately striking forward with e5. He has time to survive 😭