As everyone has mentioned, we stopped supporting Dynamics GP 10.0 back in Oct, 2017, so it's curious as someone is still selling it.
The 'sa' you are being asked for, is the system admin account that is created when SQL Server is installed.
You'll need a SQL Server instance installed before you can really do anything with Dynamics GP, as you'll need to create the system and company databases for Dynamics GP onto SQL Server, through GP Utilities.
If you have questions, you can open a support case or continue to ask them here.
Thanks