The latest ISM Revit plug-in version 11.04.00.22 doesn't work with Revit 2020 or any earlier version of Revit..
You can use earlier versions of ISM Revit Plugin for earlier versions of REVIT. Just similarly install an older version of ISM Revit Plugin that is compatible with whatever version of Revit you want to target. If the version of ISM Revit plugin is unavailable on software downloads we can make arrangement to make the older version available to you.
REVIT has its own release every year which includes drastic API changes, our policy remains to support the last 2 versions of REVIT..
ISM Revit Plugin Support Solutions