MissingComponentException: There is no ‘Camera’ attached to the “First Person Controller Prefab” game object, but a script is trying to access it.
You probably need to add a Camera to the game object “First Person Controller Prefab”. Or your script needs to check if the component is attached before using it.
FPSWalker.Start () (at Assets\Standard Assets\Scripts\FPSWalker.js:43)
Component>Rendering>Camera 
Yes writing AI without machine-learning is hard. But ML-Agents is not just simply "hard to set up". You cannot even apply ML-Agents to all projects. There are issues that it will never learn to do. And even if it is applicable you have to know what you are doing. Designing an observation space and a reward function is something that has so many pitfalls that you have to know what you are doing. Not to mention the configuration of the learning process. Please don't try to encourage beginners to use ML-Agents.
– Captain_Pineapple
The error message says it all.
– DaveA