To start building apps targeting Universal Windows Platform, make sure that you have the Universal Windows Platform development workload installed on your Visual Studio 2017 dev environment.

Open the Visual Studio installer from the start screen or run the setup from the control panel and then click Modify, as shown in the following screenshot:

This will launch the Workloads screen. Check whether Universal Windows Platform development is checked. If it is unchecked, it means that the components related to it are not installed. Check the workload and click on Modify to install all the required components. It will start downloading the required packages from the Microsoft server and complete the installation:

If you have already downloaded the offline installer, alternatively, you can run the installer from the installer path to save the bandwidth. If there are any new updates available in the server, the installer will auto download them.