Click on preview, and there you should see something like this.
First, you have to download Expo Client on your phone (iOS or android). When you complete your download, click on preview app and it will open in your browser the app that you should scan from your phone with the app that you already downloaded (Expo Client).
NOTE: If you are on iOS device and you are not able to scan, open your camera and scan the QR or login into the Expo Client app with this username and password
On your phone will open the UniExpo app. And click on the scan icon next to Choose an app to preview. Look at the picture below.
After that scan the QR that you already have into your app builder.
Now you should be able to preview your work.
NOTE: Your initial screen must have data. Otherwise, the app will stick on the loading screen.