Mistake that I don't understand

1) d4 d5
2) c4 dxc4
3) e4 e5

White to move, I would move d5 but this engine says that is a mistake, why?

Because white is down a pawn and he has to play energetically to develop initiative and regain the pawn - Nf3 followed by Bxb5 is the correct way. By playing d5 you lose a tempo, black has enough time to play b5 - securing the pawn and also develop his pieces with tempo like Nf6 - easy game for black. If you do not like complicated lines - play 3.e3, where after 3...e5 4.Bxc4 is possible. In the 3.e3 line 3...b5 isn't possible as well, you can check it with the engine 4.a4 is giving white huge advantage

ok thank you, I sort of grasp the concept of tempo and complicated lines but sort of not, Youtube here I come lol

Just play 3 e3 its much more simple :) and u get a good game . 3e4 is good but its complicated and u gotta know theory .

Still simpler, you can play 3.Nf3 to prevent e5, and then 4.e3. This is actually the most popular continuation at all levels. It is also a very instructive line of development.