How do I add a menu task to /node path?
What you have done is to create a menu task on the route entity.node.canonical with the title of Import JSON from file which links to the route entity.node.import_form.

The error you are getting is because the route entity.node.import_form (the link of the task) doesn't exist. You have to either depend/enable the module that provides this route or create the route yourself.