Not so much a bug as a question of whether or not this is intentional. I can't make a signboard without discovering a sketchpad. If this is an intended mechanic I don't mind, I just find it odd because all other buildings that can have a sketchpad added have it as optional.
"The psychological trials of dwellers in the last times will be equal to the physical trials of the martyrs. In order to face these trials we must be living in a different world."