I managed to purchase a Husbandry Arm from a Trader, but I was unable to install it. I checked the code and it said that I didn't need anything to install it except the Mechanoid Hacking Basics Project. The Husbandry Arm Project only involved building your own Husbandry Arm, NOT installing an existing one.

I was unable to install it on my Mechanoid, so instead of force-researching the Husbandry Arm Project, I used cheat codes to add its Hediff to my Mechanoid. That failed. I closed out everything and restarted my game, and that also failed. The Mechanoid does not have the Work Ability to tend to Animals in the Hacked Mechanoids UI.

However, I was able to uninstall the Husbandry Arm just fine, and get it back.

I am betting that the answer is obvious: I need the tech to install it as much as I need to build it, but that is clearly not what you intended when you commissioned this Mod, otherwise the Modules would probably not be for sale and the code would explicitly require the given Module.

Okay, so I researched the Husbandry Arm technology and tried to install it. When I went to look, I was given the oppurtunity to install a Hauling Arm or a Self Destruct Button, which were both researched and in my possession. I can install and uninstall Modules such as the Vanometric Modue, which I previously added. There is no option for the Husbandry Arm.

What am I missing? Am I not skilled enough to perform the surgery?