This key is used for server side purchases verification. Obtaining the key is a bit tricky but you'll do it only once. You must be the owner of the project to proceed.
2. Select API access.
3. Click Link to connect your Play account to a Google Developer Project.
4. Under Service Accounts section click Create Service Account.
5. Click the link to open Google API Console.
6. Click Create Service Account.
7. Enter the name of the service account.
8. Set the role to Project Editor.
9. Click Create Key and download JSON. This is the key you will upload to Adapty Dashboard, but you're not done yet;)
10. Go back to Google Play Console (API Access) and click Grant Access on the service account that you've just created.
11. Grant View app information, Financial Data and Store Presence permissions and create the user. That's it, don't forget to upload JSON file to Adapty Dashboard.
12. Optional. It takes at least 24 hours for for changes to take effect but there's a hack. In Google Play Console, open Your App Dashboard -> Store Presence -> In-app Products. Change description of any product, click Save. Everything should be working now, you can revert in-app changes. If not, make sure Google Play Android Developer API is enabled in Google Cloud Platform API Library.