I had a bug where the "timer" in the last quest doesn't work if I shoot out the spotlights. It caused it just to go into an endless cycle of the bots keep coming, both times I shot out the spotlights in the final fight. I had to do it 6 times because it kept glitching up or freezing when I tried to leave from the nearest exit (after the fight and story).
One of those times I ran into that error message you were talking about. As I simply thought there was something I was supposed to be doing to get the final fight to continue. I tried building a robot at the station there, figuring I needed to use it to build a robot, to get the mechanist to come from their hiding area. It kept showing the error you mentioned. But it eventually let me build a new robot. Which was pretty crappy as the area wasn't attacked to my supply line yet. And it was literally build with the junk your carrying situation.
But in hindsight I should of never been able to build that robot during the final fight.
You problem on the other hand could be a huge issue. But keep trying as it just seems to give that error, erroneously several times when building a robot. It does it sometimes not always, and every time it said it for me, I eventually built what I wanted.