You don't actually need cli to get the ARN of your S3 bucket. In the management console select the bucket and in the pop up that opens you have the option to copy the ARN if the bucket you choose.
In fact, you don't even need to do that. If you know the name of the bucket, you know the ARN. No need to 'get' it from anywhere. It's always arn:aws:s3:::NAME-OF-YOU-BUCKET.