The K.K. concert is a scripted event that plays the next day after Isabelle reads K.K.'s review. After the concert, your player walks out of their house to start the day, when they are immediately greeted by Tom Nook, who installs the terraforming app on your phone. At that point, I believe all you can do is make simple dirt paths.
You must now go to the ABD in the Town Hall and buy your licensing for Waterscaping and Landscaping. You can also buy more pathing options, including the option for custom design pathing.