Replying to this thread because i found it on Google, while researching the same problem. If someone like me stumbles here too, there is a solution.
What happens:
somehow maya locks default shading group node, this prevents material assignmnts and, by extension, creation of input nodes for poly primitives.
What you need to do:
Unlock default shading node
Mel command to unlock initialShadingGroup:
This problem is present up to Maya 2019, maybe even later.