[2020] New Mods won't work correctly

Make sure you're using the right tools. Industries and Campus has their own area tools, which are similar to districts, but also slightly different. The best way to go about it is to paint in the area with the appropriate tool, then place a main building to set the type. Then you will be able to place buildings in the area.

There's the Industries area tool highlighted. The Campus one is the right-most icon.

https://i.imgur.com/3Nv77k6.jpg