There had been a few incidents where some sensitive data was stored in S3 and accidently the S3 Bucket was given public permissions. To avoid such incidents, AWS has introduced an additional step which has to be performed before making the Bucket public.
Go to the Permissions tab for the S3 Bucket. Click on Edit and unselect “Block all public access” and save the settings. Now the S3 Bucket can be given public permissions through ACL. This is the additional step to be performed.