I have the same issue. I have solved all puzzles but 12, and do not have any builders. I am not at the pop max of 65, i have 60, but am wondering how new villagers would learn construction. Is this puzzle not solveable by the GC?

The GC is a child and cannot perform 'ordinary work,' which is done only by villagers who are at least 14 years old. Builders can be trained in v1.1 by having them repair the huts that they have built.