I ran across this error for the first time today and wanted to post what ended up working for me. Based on other posts I assumed that the file path length was my problem. I typically just remap my project folder to a new drive letter and that solves the issue for most GIS tools. It did not help here. I finally moved my Geo-RAS project folder to my root C drive and then the floodplain delineation worked as it should. Hope this solution works for others as well.
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