Why is Kaine glowing with a green aura

Why can't I give Yonah a pumpkin?

This happened to me as well. Since you have reached the part right before the point-of-no-return, then unfortunately you can no longer complete this side quest. This quest (as well as Yonah's Cooking) must be completed shortly before the point of no return. Since the quest is not completed, it should appear under the "Incomplete Quests" section in your quest logs.

Story/lore wise, during this part of the story, you should have learned or at least will about to learn that

Yonah's condition has worsen, and you are tasked with finding medicine for her. Since she is extremely sick, she does not have the energy to take part in the events of those quests.

How do I complete Yonah's Gift before the point of no return?

At this point, the only way to complete this side quest is to replay through Part 1 - which only be done by starting a brand new game or make your way through the story and unlock the requirements needed for Route E.

Route E also requires you to start a new save file, but branches off into a secondary storyline that follows Kaine