Your commander cannot build these things, you need a level 1 construction unit (Kbot, Vehicle, Aircraft, etc) to build a geothermal plant, and a level 2 construction unit to builder a reactor and moho mines.
Ideal steps:
Use commander to build basic a few solar panels, a Kbot lab, and metal extractors on nearby metal patches.
Build a Construction Kbot from the factory.
use construction Kbot to build an Advanced Kbot factory.
Build an advanced Construction Kbot
now you can use the Advanced construction Kbot to build a reactor or moho mine (or big lasers!!!).
You can of course to similar things with aircraft and Vehicle plants.
Hope this helps :)

Edit: oops, I've just realized what you mean now, yes, you are restricted to building a Kbot lab, metal extractor, Solar panel,s, and light lasers. Use the left and right arrows below the build window to flip between pages, Kbot lab is on the second page.
Once you have the Kbot lab built, you can select it and start building new units from it.
You can do this from mission 2 onwards of the Arm campaign.