generative modeling for Rhino
It's not so much an error as a warning. Errors are red, warnings are orange. My guess is you didn't put a function into the component. You can assign a function either by double clicking the Function component or by writing the function on a Text Panel and plugging this panel into the F input.
ps. If you mouse-hover over the orange balloon in the top right corner of the Function component, it should tell you what the warning(s) are.