To run your application, click 'Build & Run' (Play Icon) on the top right corner of the screen. Clicking this button builds your application and generates a .apk file for an Android application or .ipa file for an iOS application and gives a QR code or link to download the app.
The build logs while the application starts to compile and execute in a dialog box. Scan the QR code or use the .apk/.ipa link in your mobile to download and install the application.
Configuration mismatches, improper parsing of your input, your configurations not saving are few reasons which might lead to build failure.
Note: If you wish to preview your application on an iOS device, remember to enter the UDID code which is specific to your device. This action is not required for an android device.