ArcGIS 10.8.x is the final version of the Esri ArcMap product line. HEC-GeoHMS versions and 10.3.x are known to work with ArcGIS 10.8, but require careful installation due to changes in Python, .NET Framework, and licensing.
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You must have Arc Hydro Tools for ArcGIS 10.8 installed before installing HEC-GeoHMS. ArcGIS 10