Final answer:
The error message "Constraint rule on a hidden attribute" occurs when a Constraint rule tries to apply restrictions on an attribute that has already been hidden.
Step-by-step explanation:
The error message "Constraint rule on a hidden attribute" occurs when Constraint rules run first and the attribute values are hidden before a Hiding rule can be triggered.
In this scenario, the Constraint rule is trying to apply restrictions or conditions on an attribute that has already been hidden based on a Hiding rule. This can lead to conflicts and result in the error message being displayed.