Hi Harshitha,
To answer your first question: the “Build Native Mobile App” option being greyed out usually means your app does not have a Native navigation profile added. If you want to create a Mendix Native app then creating a Native navigation profile, building your project using the Mendix Native Mobile Builder and then further building your actual app is your only option. Keep in mind that the Mendix Native Mobile Builder creates a React Native Project, not an "app" (ipa/apk) that can immediately be installed on a device. Read more about how to do this here.
However, if you just want to show a single screen to mobile users there are other options. You could create a web page and make that available exclusively for your mobile users. An option here would even be to create a Progressive Web App (PWA). PWA's are an evolution of traditional web apps that tend to behave more like native mobile apps. Read more about them here.
If you share more about what you are trying to do I can help give more specific advice.