AWS DMS endpoint test connection error
I had a similar issue, though I was using DMS to migrate data from S3 to an (RDS) SQL Server 2008. I figured out what my issue was, I didn't require SSL on the SQL Server endpoint. I additionally had to make sure my IAM policy had the right level of access to the services. Hope this helps.

Additionaly:


  AWS DMS currently does not support SQL Server Express as a source or target.


Reference: Troubleshooting Migration Tasks in AWS Database Migration Service (AWS Database Migration Service | User Guide)