quick fix

src/controllers/collectionController.js

@@ -50,14 +50,14 @@ exports.createCar = async (req, res) => {
       await userCollection.save();
     }
 
-    let user = await user.findById(userId);
-    if (!user) {
+    let founduser = await user.findById(userId);
+    if (!founduser) {
       return res.status(404).json({ message: "User not found" });
     }
 
     // update number of cars in the collection
-    user.number_of_cars = userCollection.cars.length;
-    await user.save();
+    founduser.number_of_cars = userCollection.cars.length;
+    await founduser.save();
 
     // 7) Return success
     return res.status(201).json({