I just came across this issue with ArcGIS Pro 2.1.2. I haven't attempted to publish anything in a month or two (probably since last update). However, I did come across this help article...
Error: External sharing process failed
According to the article I must have a separate Python install, which I don't appear to. I have neither the Environment Variables for it, nor is it listed in Apps & Features (Win 10). The first two fixes pertain to the Python Env Vars. The third recommends zipping a shapefile and publishing it directly to ArcGIS Online (I'd rather not since the source data is an FGDB FC). And the final fix is completely uninstalling/reinstalling ArcGIS Pro, but who has the time.
I will try publishing from ArcMap if I can't figure it out.