Postgres export to S3

I'm trying to export a snapshot to S3, but nothing actually gets exported and the export metadata file shows a PERMISSIONS_DO_NOT_EXIST skip error on every table. The docs say this is a permission error that can be resolved by granting permissions to the tables:
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_ExportSnapshot.html#USER_ExportSnapshot.postgres-permissions

However, I've done just that with my master user multiple times and nothing has changed.

Basically the whole export metadata file looks like this:

{"perTableStatus":[{"tableStatistics":{"extractionStartTime":"Apr 17, 2021 10:27:49 AM","partitioningInfo":{"numberOfPartitions":1,"numberOfCompletedPartitions":0}},"warningMessage":{"skippedTable":[{"reason":"PERMISSIONS_DO_NOT_EXIST"}]},"status":"SKIPPED","sizeGB":1.8310546875E-4,"target":"mailserver.public.account"}

Am I doing something wrong?
Any help would be appreciated.