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

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.

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.

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
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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.
For example com.myapplication.subscription.premium.monthly

This identifier must be entered in Purchasely console as well, you can follow our documentation for more details

Create a paywall in our console with a button to your created plan and display it in your application.
If you build your application with the same app package id and version code as the one you uploaded to Google Play Console you should be able to make a purchase as a tester