First thing to note is that a Linux user (e.g. root above) is not an IAM/AWS user. An IAM user can be created and authenticated through an ID and private key. (And I assume you'll want to avoid using your root account for any such purposes)
When you go to the S3 manager, you should be able to click on the bucket and then edit the permissions. From there, you should be able to use the AWS policy generator. There are numerous sample policies online as well.
Best practice is to add permissions to a group of users, not just one user. At first, you may find that you are not providing overall permissions that are required, so it is coming back with an error.