Find and print the total number of menu items and the average price of the menu items for each restaurant
You are missing GROUP BY: 

SELECT r.restaurantName, COUNT(m.itemNo), AVG(m.price)
FROM Restaurant r
INNER JOIN MenuItem m
ON m.restaurantNo = r.restaurantID
INNER JOIN Dish d
ON d.dishNo = m.dishNo
GROUP BY r.restaurantName;