how to setup and configure pub/sub extension

I follow this doc https://docs.apigee.com/api-platform/tutorials/add-extension/add-new-extension

to configure extension so i did the same

1. Add extension named as pub_extension

2. then configure and deploy extension(using service account)

3. then follow this doc and configure the same proxy

while i trace the api i got 500 status code error and null in messageID

11042-time.jpg

and got the log in extension tab as below

Request #rrt-28742469795008600-c-gce-11152-511884-1 - Status 403 - Error thrown: Error: 7 PERMISSION_DENIED: User not authorized to perform this action.

i granted role as a pub/sub publisher

can you help or guide how can i authorized

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