Why does it say The type or namespace 'BoardManager' could not be found. Are you missing 'Completed' using directive?

[JollyApp]

I was watching part 5 of the tutorial 2D Roguelike tutorial and I got the error:

The type or namespace ‘BoardManager’ could not be found. Are you missing ‘Completed’ using directive?

I had all of the code from previous videos and that video.

Here are some screenshots of the error:

[UnityCoach]

It tells you that BoardManager class/type is unknown.
If you have a BoardManager class, it may be declared inside a namespace. If so, you must add

using nameofthenamespace;

at the beginning of your script.