I was able to run through the RAS postprocessing on ArcMap 10.8 successfully on my test dataset. Couple of suggestions:
1. Make sure you have the right version of GeoRAS installed. Latest for ArcMap 10.8 is GeoRAS 10.8.0.3. If you have to install newer version, make sure you first uninstall the previous one.
2. Start a new project and do the import again (that sometimes does the trick).
3. Use raster representation of the elevation model (not TIN). You already did that to no avail so check below.
4. Try to use larger cell size for your DEM generation - might be a disguised memory issue. Try to keep the DEM under 10,000 by 10,000 cells (or much smaller initially). If that works, you can then run it with smaller cell size (larger DEM) until it stops working.
5. During both import and water surface generation and floodplain delineation, GeoRAS provides a long list of steps it is going through - do you get those before the error message you posted appears? If so, can you see at what step the tool is failing (copy/paste here)?
5. Do the delineation by hand following these steps:
a. Convert water surface TIN to raster to create a water surface raster. Make sure the cell size, extent, and snap raster environments are the same as of your DEM (convert terrain TIN to DEM first if need be). For reference, I'll call them DEM and WSEG
b. Subtract DEM from WSEG (WESG - DEM). That will generate raster where positive values represent depth of flood while negative values are "useless". Call it WDA (water depth "all").
c. Use "con" statement to remove all negative values form WDA and replace them with "no data". That will generate a water depth grid with only positive (meaningful) values. Call it WD (water depth). If you do not need the water depth raster for later analysis and visualization, you can skip this step and go to the next one.
d. Use another "con" operation, but this time around, convert all positive values in WDA to an integer constant (let's say = 1). Call it WE (water extent) grid.
e. Convert WE to a polygon. You can do smoothing during this step.
f. If desired, you can do clean-up on the polygon(s) - e.g. remove non-directly connected (to the river) floodplain elements, do additional smoothing and generalization, etc.
I also suggest you review the "Arc Hydro - Project Development Best Practices.docx". All those suggestions work well for GeoRAS project development. Section 6 has some additional things you can try.
Let me know how it goes. If none of these steps work, we might have to look at your data.
(If you can, please move this post to the "water resources" GeoNet topic. H&H and "hydro" in general type of questions will get better visibility there).
