Good question @pat - this is not a bug. Plan types must be added in code provided by a module, we do not support creating them via the UI. Perhaps we should consider adding a message to this effect at both /admin/structure/plant-types and /plans when there are no plan types enabled.
As mentioned here in the docs farmOS core does not currently provide any plan types: Plans | farmOS
There are grazing and crop plans available for farmOS 1.0 but they have not been ported to farmOS 2 yet.