In HEC-RAS:
Ensure you download the HEC-GeoRAS installer specifically labeled for "10.8." Versions designed for 10.2 or 10.5 will likely cause "Library not registered" errors. hec georas for arcgis 10.8
Use ArcToolbox > Data Management > Calculate Geometry to add Length or Station values automatically. In HEC-RAS: Ensure you download the HEC-GeoRAS installer