The tree of da’as of good and bad held the new knowledge that good is bad and bad is good. Physical good (temptation etc.) is spiritual bad and physical bad (toil etc.) is spiritual good.
Chava was thus surprized to see that the tree was physically good and yet prohibited, because she thought that good was good and bad was bad.
They should have gained the tree’s knowledge without eating from it, but instead they gained its knowledge in sin. And now it was clear that sin was desirous, and sin subjugated them to the desire for bad things. This caused death.