Error message:
google.api.core.exceptions.Forbidden: 403 GET https://www.googleapis.com/storage/v1/b/transcribe-test?projection=noAcl: postcast-cyberdyne@podcasttranscription-184110.iam.gserviceaccount.com does not have storage.buckets.get access to transcribe-test.
Solution:
I had the wrong bucket name in my script. After changing it to the project name I had chosen, it worked.