Having watched and following some tutorials on how to enable In-App-Purchases in my app, I've noticed some preparation steps are missing from them. I've decided to include them in this little post, to help you minimise the number of apps you upload.
So as most tutorials would have said, you need to upload an .apk (or app bundle) to Google Play Console, to start setting up the "In-app Products".
However, what most fail to explain, is that your app needs to have the Billing Library added before Google Play Console would allow you to add In-app Products.
So in the case of using flutter, you'd want to include the in_app_purchase package to your pubspec.yaml file, even if you are not ready to accept In-App Purchases just yet.
A few additional steps you should take, is to update your Policy > App Content > Content Ratings.
Within the Miscellaneous Section, there is question "Does the app allow users to purchase digital goods", and you should check the box "YES".
You should do this as long as your app includes the billing library, even if you haven't started posting anything for purchase, and I usually update this before uploading the the new .apk (or app bundle) with the package installed.
Once you've done those steps, and uploaded your new .apk (or app bundle), allow some times for your changes to be reviewed. Once reviewed, you should be able to add In-app Products to make available for purchase.