Okay, in order to make it simple for you, it’s because you don’t have the android SDK installed - or up to date, in your computer.
You can start by downloading Android Studio and install it. Launch it and start a new project (Follow the default process, we don’t care). Once on the project, launch the Sdk manager (see here).
On the first tab, select the latest Sdk platform (7.1.1 currently). on the second tab, select Android SDK Build-Tools, Android SDK Platform tools and Android SDK tools.
Install all this, restart your terminal, and you’re done