If you are exporting an EC2 instance to an S3 bucket using
aws ec2 create-instance-export-task, and you are receiving an error message like the following:
Access denied to the bucket <bucket name>
then you may need to add a permission to your S3 bucket granting permission to AWS to write to the bucket.
According to the AWS documentation Exporting an Instance as a VM Using VM Import/Export: Prerequisites, you must attach an access control list (ACL) to the S3 bucket. The exact ACL depends on the region in which the EC2 instance resides.
The ACL added to my bucket in Oregon (us-west-2) looks like this: