It is due to the region is missing. For init script saved in S3, the region field is required. The init script section should be like below :

"init_scripts": [
            {
                "s3": {
                    "destination": "s3://<my bucket>/init.sh",
                    "region": ""
                }
            }
        ]

Note that the regions value can be empty " " or the specific region you want to configure.