This month, one of our Revit-using clients received this error message when trying to install Revit hotfixes (c'mon, Autodesk, let us automate hotfixes already!)
Eventually, my Google-fu brought me to this article on the EA support forum, which pointed me to the solution: the "QT_QPA_PLATFORM_PLUGIN_PATH" system variable was missing.
I pointed that system variable to to "C:\Program Files (x86)\Autodesk\Autodesk Desktop App\platforms" as it contained the qwindows.dll file and it was a folder that was product- and version-agnostic.
After adding that system variable and rebooting, which Windows requires to read-in system variables, the user could install the Revit 2021 hotfixes via the Autodesk Desktop App! 🙌