Rather than showing us the code, please show us the exception that you are getting.
The point is that you are using tf.keras.Sequential and it takes an argument, which is a python list of instantiated functions. Well, they mention that you can also use the .add() function, but just making a list is simpler. So you need commas after each one. The syntax matters.
Here’s the recommended thread to read for help with both of the sections of this assignment.