Setup Sysadmin role in RDS SQL Server

the 'sysadmin' role itself is reserved for the 'rdsa' internal SQL Server account, which is managed by the Amazon RDS service and not accessible to users.

you can create a new user and assign db_owner role

CREATE LOGIN [NewUserName] WITH PASSWORD = 'NewUserPassword';
GO

USE [YourDatabaseName];
GO

CREATE USER [NewUserName] FOR LOGIN [NewUserName];
GO

EXEC sp_addrolemember 'db_owner', 'NewUserName';
GO