You need to have CADmep 2019.1 installed which means you need to have AutoCAD 2019 installed. It's using the 2019.1 Fabricaiton API DLL's.
They may be supporting 2020 now in the last update. It was updated before than rolled back due to an Autodesk issue and I think it's now rolled forward again. I've not retested to verify aside from noticing the first time 2020 was supported that caused a few public data issues.
I don't believe you need a license of CADmep, simply a demo installed should be enough. But you likely have it anyway of you have an AEC collection.