The NavMesh (short of Navigation Mesh) needs to be generated before any bots can spawn. In MvM, the population file simply won't load without it.
So, typing nav_generate in console will generate a basic nav mesh for robots to move and navigate the map. Then the pop file will load, and robots can spawn.
If you're getting errors in your pop file, you need to make sure all the brackets are closed, and make sure there are no invalid commands or parameters.