Update from 7th of August 2025:

I have a firebase cloud function for handling RTDN(real time developer notifications) but i got an error message that i don't have the required permissions. AI tools could not really help me all the way. With the info I could get from them and the answeres from people in this post that I am writing this answer for I ended up getting it to work like this:

In the google cloud console under Navigation Menu -> IAM & Admin -> IAM i searched for an entry with a Principal like this "[email protected]" and a name like "App Engine default service account".
Then i went to the google play console app-list screen -> Users and permissions -> next to "manage users" there are 3 vertical dots -> clicked on them and selected "invite new users" -> for email address i entered "[email protected]", under account permissions i only chose "View app information and download bulk reports (read only)", "View financial data, orders and cancellation survey responses" and "Manage orders and subscriptions" and pressed the button to invite the user.

Then in the google play console i went to the app in question -> to the subscription (in my case i only have 1 anyway) and deactivated and reactivated it and after a few minutes it worked for me.

Hope this might help someone in the future.