I don't think the supply agreements come into play here as long as your settlements are linked with the caravan services. As for the rest...
Find the room you "built" the power transfer room in at HQ. For me, it was the room in the basement with the elevator, there may be another option that you used, I don't know.
1. Is it clean? If no, assign your goons to clean it, but choose cancel when asked what to build.
2. Once clean, while in the room use facilities>build>(that room's name) wait until it prompts you to choose what to build. For me, I only had the one option. It should eventually pop up with the list of things you have to have to build it. If you have it all, accept it and you're good to go. You should see the construction supplies spawn somewhere nearby. Those will tell you that it's working.
3. Leave HQ for a few days (with minimal waiting/sleeping as this just compiles the background shit the game is trying to do). I like afking at home plate since in barely loads anything else. Really, just go there and do something else for a couple hours while the game processes everything. Just make sure it doesn't pause.
4. Go back to HQ and check the room. If it's ready you should see a number there now. For me it was only 850/1000 and that is where I'm currently stuck, but the point is progress, not necessarily completion. If the room is still empty, use the wall panel for that room(for me, it was next to the elevator) and refresh the room, then go into the next room and wait(afk again) Depending on your computer this part can take awhile. I did this a few times before it worked. If this doesn't work for you, you'll need someone smarter than me to help you.