Belly cannot join the Paris exchange program in The Summer I Turned Pretty season 3 because there is no room for her.
This occurs in episode 9, “Last Call,” after Belly boards a flight to Paris following the cancellation of her wedding. Once there, she learns that the exchange program has no available place for her, leaving her devastated and uncertain about what to do next.
Instead of immediately returning home, Belly remains in Paris. She finds work, continues her college classes online, and gradually builds a new life among people she meets there. The exchange-program problem therefore becomes the practical reason her Paris storyline changes direction.
The issue is not a stolen passport, although Belly’s backpack is later stolen at a café and contains her passport and engagement ring. The travel-document theft is a separate event from the lack of space in the exchange program.