Help! Why could not I use my pawn to kill bishop in this move?

You aren't allowed to put yourself in check. It's against the rules.
So exd6 is an illegal move, because you would be putting your own King in check by his Queen.
You aren't allowed to put yourself in check. It's against the rules.
So exd6 is an illegal move, because you would be putting your own King in check by his Queen.
That would leave your own King in check by his Queen.