backend/src/main/java/com/petshop/backend/service/CartService.java

@@ -246,7 +246,7 @@ public class CartService {
             Cart cart = cartRepository.findById(cartId)
                     .orElseThrow(() -> new BusinessException("Cart not found"));
 
-            if (!cart.getUser().getUserId().equals(userId)) {
+            if (!cart.getUser().getId().equals(userId)) {
                 throw new BusinessException("Unauthorized");
             }