I’m a big fan of ProMash, and have been using it for many years. the other day i tried opening a help file and got an error message that help is not supported. Error opening Help in Windows-based programs: "Feature not included" or "Help not supported" - Microsoft Support
Does anyone have a fix for this?
If you install Linux it works fine under Wine. 
I just heard from someone using Promash with Win10 who said he had no issues at all. I know that doesn’t help much, but it shows it can be done.
Same happens under windows 8.1.
Solution that works for me: Snag a copy of “winhlp32.exe” from any Windows XP computer (C:\Windows) and move it to your Win10 system someplace. Run it and it will prompt you for which help file to open; navigate to your promash install directory and select the HLP file. Not ideal, but about as good as it’ll get until the software gets updated (if that ever happens). Legacy software = Legacy solutions 
The cheap and easy fix is a pen, spiral notebook and TI series calculator. Anyone seen my slide rule? ;D