I assume it's because this requires technology or ingredients that don't exist in the Ultracube modpack. I'm unsure which mod's responsibility it is to add compatibility (assuming that's something the mod author wants!) so I'm mentioning to both.

Hello! Thank you for report
Please send me an example map (or save file) with bug description, so I can see if it fCPU bug or not.

On any Ultracube save, execute /c for _, tech in pairs(game.player.force.technologies) do tech.researched=true end to unlock all technology. Then observe that the fCPU requires Processing Units to craft, which are not a thing in Ultracube. Is this a compatibility issue that fCPU should handle, or Ultracube?