Hello and sorry for the late reply due to site migration.
It seems like that there is another asset (or your own code?) in the project, that has a class named “Action” which is though not put within a namespace and as such it creates a conflict with the “System.Action” type.
Can you please confirm that this is indeed the case and/or that importing NodeCanvas on a new project works correctly?
If it is due to another asset/plugin, I would suggest to kindly ask its developer to place his code in a namespace, since that is a standard practice to avoid such conflicts.