I can't find Build Web option

I downloaded the free uniny engine to make games. When I go to the build settings to create a platform, web builder is not an available option. What do I have to do to make web builder an option. I’m following the tutorial through the space shooter game and I want to know so I can progress in the tutorial

You need to specify that you want the WebGL build platform installed when you run the download assistant: Unity - Manual: Installing Unity from the command line