How to change message type from warning to error on resignation form

Hello Everyone,

In Oracle when an employee resigns, their last working day is auto-defaulted based on the Notice period entered e.g. notice period of 30 days and resignation date is 07-Jun-2024 then Last Working date defaults to 06-July-2024. The field shown is editable but if user enters a date less than the notice period e.g. 03-Jul-2024, system throws a warning message (Screenshot attached). I want to convert this warning message into an error so that user may not be able to proceed further if their last working day is less than the defaulted value. Kindly guide on how to do so.