Navmeshes cannot be generated at runtime. The NavMeshBuilder API is edit time only.
Since Unity 5.0, there’s no difference in the engine features between the pro version and the free one. Instead, pro users gets access to a bunch of extra services, like analytics, cloud builds, and so on.
If you need a navmesh solution that can build at runtime, you should look into the A* Pathfinding Project - I believe that’s able to do those kind of things. It’s a popular product that’s got a good reputation in the Unity community, and comes in both a free and paid version. I don’t have any personal experience with it, though.