Player A attacks with Grizzly Bears. In second main he plays Chart a Course. Opponent play Twincast, targeting Chart. Does the opponent discard to chart?

I am getting reports of that rulings are that Op does not discard, both tournament ruling and Mtg Arena.

The opponent must discard a card, because they did not attack with a creature this turn.

As a spell resolves, its controller follows all of the instructions laid out in its spell abilities, and Chart a Course's spell abilities tell its controller to discard a card if they haven't attacked with a creature this turn. Player B hasn't, so they must discard.