@Sindhusha Seelam 

To show a custom validation message in a Constellation app, you should add the error message directly to the field you are validating, not at the page or data view level. In your Request Data Transform, use the “Add Message” action on the specific field like .FieldA or .FieldB where the error should show. This helps the UI display the exact message next to the field. Avoid adding messages on read-only data pages or using general page-level messages, as Constellation will then show a generic “Data view has message error.” If you want to show a banner message at the top, you can add the message to pyCaseMessages() on the main case page. This way, users will clearly see your custom message instead of the generic error