When you open the tutorial file, there are two artboards. To view both artboards at once choose View > Fit All in Window. The artboard on the right contains resources for you to use during the tutorial, and the artboard on the left is where you will be working. Open the Layers panel by going to Window > Layers. Click on the layer named "Rectangle."
Press the "V" key to choose the Selection tool and click on the pink Live Rectangle. Notice that there is a circle in each corner of the Live Shape. Click on one of these Rounding Widgets and drag it until you see a red stroke on the left and right sides of the shape. This indicates that you have reached the maximum corner radius.
That's it! You're done. Please provide feedback in our survey so that we can make sure this tutorial is as useful as possible.