To be able to purchase an in-app product with Google using Purchasely SDK you need to follow some steps as described in Google documentation
Create a Google Play Developer account
If you do not have an application or an account on Google Play you need to create one here and follow the steps.
The process is fast and requires a lifetime payment of $25.
Upload a version of your app with Purchasely SDK
To enable in-app purchases with Google, you need to upload a version of your application (APK or Bundle) with Google Play Billing SDK.
Purchasely SDK already integrates Google Play Billing, you can add the core and google dependencies as described here to your application to enable the feature.
Upload your version on an internal or closed track for a fast review and test it almost immediately.
Create your products
Once your application validated by Google, you will be able to create in-app products (consumables and non consumables) and subscriptions.
Select the type you want and click on Create product in the top right corner
Choose an identifier for your product, we strongly encourage you to use a model with your app bundle id since the product id must be unique across all your applications.