Unable to upload to S3 while using amazon polly.

Do the following steps to troubleshoot:

1. Go to S3 and verify in which region the S3 Bucket you're using was created.
2. Access S3 Bucket you're using and click on Permissions, then scroll down to Bucket Policy and verify if there's a policy attached that's blocking your access.
3. Go to Polly and verify in which region the Polly instance you're using was created.

to expand on @vtjean comment answer with links to the documentation:

• you need to call StartSpeechSynthesisTask with an IAM user or IAM role that is allowed to access Polly and to write to S3 bucket you specify. See https://docs.aws.amazon.com/polly/latest/dg/asynchronous-iam.html for an example of the policy that can be used for that purpose. For a guide on troubleshooting S3 access see https://docs.aws.amazon.com/AmazonS3/latest/userguide/troubleshoot-403-errors.html#bucket-iam-policies.
• the bucket that you intend to use must be in the region that you use to call Polly. You can check bucket region either in S3 AWS console page or using a CLI: get-bucket-location