The rds_cdc_enable_db
method requires sysadmin
permissions that you don't have on RDS. What you receive as the master login is a custom login that AWS prepared for you and grants you as many permissions as possible without interfering with AWS RDS operations. See docs.
According to RDS documentation, you should rather use stored procedures provided by AWS in order to enable CDC:
--Enable CDC for RDS DB Instance
exec msdb.dbo.rds_cdc_enable_db '<database name>'
--Begin tracking a table
exec sys.sp_cdc_enable_table
@source_schema = N'<source_schema>'
, @source_name = N'<source_name>'
, @role_name = N'<role name>'
See also this page specifically for DMS.