When you create your project, there is an option for Maximum Quality or Scalable 3d or 2d. In my case I had set the project to Scalable. This disabled Depth of Field. It also put a yellow Scalable toggle button in my viewport on the upper left side. When I clicked it to turn off Scalable, my Depth of Field worked.
You can also find the Scalability settings in the Settings menu above the viewport.