The 32-bit architecture of ArcMap limits GeoRAS to processing DEMs smaller than 2GB. If you are modeling a 100-mile reach with 5-meter LiDAR, the tool will crash on "Build Cross Section Lines."
To use HEC-GeoRAS for ArcGIS 10.8, you'll need: hec georas for arcgis 108 upd
HEC-GeoRAS is a GIS-based tool developed by the US Army Corps of Engineers' Hydrologic Engineering Center (HEC). It allows users to preprocess geospatial data for hydraulic modeling using HEC-RAS (Hydrologic Engineering Center's River Analysis System). HEC-GeoRAS is designed to work within the ArcGIS environment, leveraging its spatial analysis and data management capabilities. HEC‑GeoRAS for ArcGIS 10