RH 2020 - Windows cannot find first topic

That's what I was driving at, your source was HTM but you were not generating that. I recommend you set File Explorer to show file extensions to make such differences more obvious.

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Our team worked through the output this morning. Discovered that if I unchecked the "Publish to SharePoint" box, it processed correctly.  Thanks for all your input. 

OK it is a .HTM file in the source but your earlier image showed this in the output. 

You have File Explorer set up not to show extensions so we cannot see what it is but Explorer seems to think it is something other than HTM based on the Type shown.

Where is the outputs folder, local or on a server?

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Can you see the First_Topic file when you view the Contents section under the Author tab in RH?

I just noticed in the screenshots in your original post the file type is ASP.NET. The preset says to look for an HTM file but that screenshot is showing an ASP.NET file. Check the file names and types.

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In the Output pane, under Responsive HTML5, Content, and Default Topic, it shows "First_Topic.htm".  When I look at the files in File Explorer, under the project folder, then the Contents folder, there is an HTML file called "First_Topic."

Adding screenshots.  

It is looking for whatever file you have defined as the default. Often what happens is you start a project and RoboHelp creates First Topic as the default. That then gets defined in the preset. Then you delete First Topic as you don't want it but don't change the preset. 

What is defined in your preset and does it exist?

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See www.grainge.org for free Authoring and RoboHelp Information