ZeroPadding2D is a subclass of the Keras Layers class, so it takes the input_shape argument. The Keras Sequential API also supports an Input function.
With the Keras “Sequential” model, the syntax is to give a list of “instantiated” layers, but not to actually invoke them with explicit input tensors. They don’t give us that much guidance about the Keras Sequential and Functional APIs (the Functional API will be used in the second part of this assignment), but here’s a great thread from earlier that walks you through how to use those two methods in some detail. Please have a look at that if you’d like a bit more information to go on here.