If it doesn't match, do not install.
https://www.hec.usace.army.mil/software/hec-geohms/downloads.aspx download hec geohms for arcgis 108 exclusive
Attention hydrologic modelers and GIS professionals! If you're working with and need to build HEC-HMS models from geospatial data, HEC-GeoHMS is your essential toolkit. If it doesn't match, do not install
To download the HEC-GeoHMS extension specifically for ArcGIS 10.8 If it doesn't match
Official HEC-GeoHMS downloads are typically found on the Hydrologic Engineering Center (HEC) website , though official support for the standalone extension has largely ceased as its features are integrated directly into HEC-HMS 4.8 and later. Download Options for ArcGIS 10.8