There are two different methods here: the Keras Sequential API and the Keras Functional API.
In the Sequential case, you only instantiate the “layer” functions and then make list with commas, as Tom pointed out.
In the Functional case, you both instantiate the “layers” and then actually invoke them with an input tensor. No commas required.
You are using the Sequential method, but you omitted the commas and the syntax that closes off the list and the Sequential declaration.
You can argue that they did not really give us much to go on here. You would probably be well served to spend a bit of time reading through this thread from fellow student ai_curious about how these two methods work.