I am using VSCode. Yep, the first error was fixed with the System.Linq, thanks!
Regarding the second error, I'm able so create scripts of my own with the NavMesh components (in the same project), its only when I try to add them to any of the Edgar scripts that i get the error.
I'm using NavMeshPlus, which is considered an experimental packade by unity, don't know if that has anything to do with it..