To build the workshop, you need:
Build the Medium sawmill and unlock the Canting machine 2.
Accept the order for delivering wood to the workshop (with the man in helmet). Please note that it must be 1st in line. Wake up the forklift driver.
When there are enough materials, find the Workshop to the right of your factory and click on the "Build" button.
When you complete the 1st stage of building, you need to build the Modern sawmill. After this, repeat the previous steps.
Your order for workshop construction may get stuck. If it is, cancel the order, and you'll get a new one. All the types of materials are suitable for building the workshop.