When I look at your city layout from 11 hours ago (elvenstats has update issues again), then I can not find a level 20 workshop in your city, but it could be under construction (in that case, you have found a bug, thiis should not be possible). Level 20 workshops are unlocked in the orcs chapter and do require orcs to upgrade. You can make orcs in the armory or collect them from orc nests which you can sometimes craft in the MA (IF they are at least level 8, so don't craft those before you are in the orcs chapter).