That means there is something missing on the previous line. Normally it would be a missing close paren, but here you are defining a list of layers. In a python list, the elements are delimited by commas, right? So did you include a comma after the previous layer entry?
Note that in the next part of this assignment you’ll be using the Functional API, so you definitely don’t want the commas between lines in that case.
They don’t give a very thorough introduction of Keras in the normal course materials. Here’s a thread that explains more about the Sequential and Functional APIs.