Took me a while to figure this one out..
Turns out the automatic install process did not add the variable to maya.env
So, long story short, in your maya.env file, look for MAYA_MODULE_PATH and add it yourself.
e.g:
MAYA_MODULE_PATH=C:\Program Files\Epic Games\MetaHumanForMaya;
That is where the MetaHumanForMaya.mod file the plugin needs lives. Change directories if you have the plugin installed somewhere else.
Note that I added the Environment variable as JamesPullan suggests a couple of posts above. You might or might not need to, I have not checked.
Hope that helped.