When a form is submitted and there is a problem, communicating that problem in a clear and concise manner is imperative. The form builder provides two ways to do this.
Error messages are displayed immediately above the offending field. They should tell the user how to fix the problem.
Additionally a summary of all the problems with the submission should be displayed at the top of the page. Every error in the summary is a hyperlink that when followed focusses on the problematic field.
This behaviour is provided by the form builder without any need for extra configuration.