Error message when generating HTML5 output

Adobe releases updates for a version to fix problems. These updates are also called "patches" so "patched up" means have the latest update installed.

Click the Help menu and check the Adobe Robohelp version number. It should be 2020.4 (The last few numbers don't matter). If it's anything less, then click Updates to run the updater. You'll need to close Robohelp, but not the updater once it's detected the update. 

Note: If it says 2020.0, you'll need to manually download the latest patch, as there was a bug in the first version which prevented updates being found.  https://www.adobe.com/support/robohelp/downloads.html

Most browsers automatically update these days, unless you turned the setting off (if available), or in some corporate environments. You can check if any updates are available in a similar way to Robohelp. For example, for Chrome, click the three dots icon then select Help > About Google Chrome. 

And is your version of RH2020 all patched up with Patch 4? Is your default browser all patched up?

What version of RH? Are you trying to view the result at the end of the process from within RH? Or have you browsed to the index file location and are trying to launch it from there?