Why: The '{pin}' input on '{node}' must receive a value — either a wire from another node OR a default value in the pin field — but it's empty.

Fix: Click on the '{pin}' input field on the '{node}' node. Either type a literal value (e.g. 5 for an integer), or drag a wire from another node's output. If you don't know what to connect, hover over the pin for a tooltip describing what it expects.