Resolved 'ActionBuffers' could not be found

Hello, I was following the document here: https://github.com/Unity-Technologi..._docs/docs/Learning-Environment-Create-New.md

But when I was almost finished, some errors occurred:

I can't find any solution about it, does anyone know how to solve it? Thanks.

 

Just now I added

Code (CSharp):

1. using Unity.MLAgents.Actuators;

and it seems ok now.