Access Denied error when trying to enable Transfer Acceleration

When I try to enable Transfer Acceleration for my S3 bucket both as the root user or an IAM user with the s3:PutAccelerateConfiguration permission, I get the below access denied error:

• My s3 bucket is in ap-southeast-2 a region which is in the supported regions list for transfer acceleration.
• The IAM user has all the required permissions: AdministratorAccess; AmazonS3FullAccess; and the s3:PutAccelerateConfiguration permission explicitly on all resources.
• The block public access setting is off.
• The S3 bucket has the below policy:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "Statement1",
            "Effect": "Allow",
            "Principal": "*",
            "Action": [
                "s3:GetObject",
                "s3:PutObject",
                "s3:GetBucketPolicy",
                "s3:PutBucketPolicy",
                "s3:GetAccelerateConfiguration",
                "s3:PutAccelerateConfiguration"
            ],
            "Resource": [
                "arn:aws:s3:::<our_bucket_name>/*",
                "arn:aws:s3:::<our_bucket_name>"
            ]
        }
    ]
}

• We have also tried the suggestion here. But that too doesn't work.

1. Why does it give the Access Denied error to the IAM user despite having all required permissions?
2. Why does it give this error despite trying to enable this as the root user (the bucket owner)?