changed field from availability to isAvailable

backend/routes/rentals.js

@@ -192,7 +192,7 @@ router.post("/", authenticateToken, requireVerifiedEmail, async (req, res) => {
       return res.status(404).json({ error: "Item not found" });
     }
 
-    if (!item.availability) {
+    if (!item.isAvailable) {
       return res.status(400).json({ error: "Item is not available" });
     }