Unfortunately, it's probably impossible for you to do the last puzzle now
It's something that you really have to plan for from the start of the game - particularly as the skill required is not only the hardest to master, but the one that has least opportunities of mastering. I literally had to allow no more than four villagers to master this skill, otherwise the others would have "used up" their training chances.
And you also need to make sure that you get someone young trained up in it - otherwise by the time you have all the tech advances, your masters might be dying off.