Build button is greyed out for Universal Windows Platforms

Hello, I have a project assignment and it's a few days left. I created a project, I installed all the required sdks, but the build button is like this, I searched but couldn't find anything about this, i am using windows 7, according to this LINK that's why it happens, but is there a way? i can't install windows 10 really my pc is old :), I know the question may be pointless but what can I do?, I need to only use Voice Recognition system. Will it work if I build it as the Default PC option?

Edit: I've researched, you definitely need to have windows 8.1 or higher :D