RDS and SQL - need to give Login db_datareader to MSDB

In Amazon RDS for SQL Server, you do not have sysadmin permissions, so you are not allowed to grant permissions on system databases such as msdb. This is a restriction imposed by the managed nature of Amazon RDS, where certain administrative features are retained by AWS to provide the scalability, reliability, and security features of the service.

For the msdb database, AWS provides a set of stored procedures that you can use to perform the most common tasks, such as managing jobs and schedules. These stored procedures are named rds_msdb, and are located in the rdsadmin

You can read the following link for more information

https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Appendix.SQLServer.CommonDBATasks.html