All official ArcGIS Enterprise software, including ArcGIS Server, is hosted on the My Esri portal.
You cannot run ArcGIS Server without a valid license file ( .prvc or .ecp ). You should generate this from the same portal before you begin the installation. Download ESRI ArcGIS Server Enterprise
: Verify your hardware and OS meet the Windows or Linux requirements. All official ArcGIS Enterprise software
Getting Started: How to Download and License ArcGIS Server Enterprise including ArcGIS Server
Once your download is complete and your license is in hand, you can proceed to the installation guide to create your server site and configure the ArcGIS Web Adaptor.