GCP CDN singed URL "ServiceException: 409 Bucket XXXXX already exists"

Question

I followed this link to config CDN signed URLs.

But get error after:

gsutil iam ch \
serviceAccount:service-[PROJECT_NUM]@cloud-cdn-fill.iam.gserviceaccount.com:objectViewer \
gs://[BUCKET]

Here is the error I got.

ServiceException: 409 Bucket [BUCKET] already exists.

I already done these steps:

head -c 16 /dev/urandom | base64 | tr +/ -_ > [KEY_FILE_NAME]
To add the key to a backend service or backend bucket:

gcloud compute backend-buckets \
  add-signed-url-key [BACKEND_NAME] \
  --key-name [KEY_NAME] \
  --key-file [KEY_FILE_NAME]

but still have the same error