I downloaded the Districting for Arc GIS and don't know how to access the extension on my portal without ArcMap.
Hi,
Districting is only available for ArcMap (not ArcGIS Pro).
Refer following link: Districting for ArcGIS | A free extension for ArcMap/ArcCatalog to configure with ArcMap/ArcCatalog.
Regards,
Sandeep B