You can only create an instant build for apps that are marked as paid. Marking an app as paid is free of charge.
Before you Install the app on your device you have to mark it as paid. Go to your app dashboard and click the Publish button.
Select an active subscription to add the app to and click Add button.
NOTE: amount per month will not be charged to you
The App is now marked as Paid and ready to either publish or Install on device.
Click the Install on device button then click "Request an Instant Build" button.
Fill in the form with your app information and upload images. Click on the Change button to upload your image and a pop up opens. Click the Upload then choose the image you like to use. If the image is not the right size, click the Resize button and your images gets resized automatically. Click Next button.
Change the images you like to use for your app if you like then Next button.
You have the option to enable or disable push notification. Next
Check permissions you want for your app. Next
Fill in the form with your Application Name and Application Package Name then Submit.
Your app is now submitted to Google Pay for instant build.
Once it's created you will see a note in your application dashboard. Click Download to install the app to your device and view how it looks before publishing it to app store.
Information of the current build of your app is found at the bottom of your app dashboard.