There might be different reasons for such behavior but all of them are caused by an inability to get data from Google. First, let's make sure Google Play Android Developer API is enabled for the project:
Visit Google developer console and select your project
Enable Google Play Android Developer API for the project
Wait a couple of minutes for the action to propagate to Google systems and retry
If entitlements are still empty, there might be a glitch on Google's side. Let's check that permissions are set correctly: if you added your in-app product BEFORE granting permission or linking to your service account, you must open "In-app products in Google Console and change something. You can for example edit the description of your product and save it. You should instantly get permission after.
