Hello,
From the Screenshot, I see that you are simply trying to create a Pub/Sub notification configuration for your bucket[0] using the Insert method. The error as on the Screenshot is a 403, suggesting that that the server understands the request but refuses to authorize it.
As you have rightly guessed, this seems to relate to your setup and perhaps some prerequisites are not met in order to configure Cloud PubSub notifications for your Cloud Storage.
I will like to point you to this link[1], which highlights the prerequisites that need to be in place to be able to create a Pub/Sub notification configuration for your given bucket. Please ensure that these prerequisites are in place, and if the issue persist, let us know.
[0]
https://cloud.google.com/storage/docs/json_api/v1/notifications/insert[1]
https://cloud.google.com/storage/docs/reporting-changes#prereqs