I have been having a problem on my server where a few players are trying to launch and hear "Can't launch without a power plant". I go to the account manager and there is no Power Plant listed in their equipment. I do a check file and it comes up with errors and asks me if I want to fix it. I click yes and that puts a power plant on their ship.
I have also seen the same thing with engines on ships that don't have removable engines.
All ships this happens to have an ID. An ID acts as a tractor beam. You can launch without an ID. You'll have your power output reduced if the server uses the technerf, but you can launch without it. You can't launch without a power plant.