If you want to run on a node but don't care where, you need to specify:

agent label:""

or

node("") ...

node (with no parameter, just a closure) is not permitted. That's deliberately blocked by the declarative subset logic.

I don't know if we can do anything about the node use case, but maybe we should create a special bareword (like none for running on any node - i.e., something like agent label: any? Not sure yet.