The NetSuite provide the facility to setup an error message if the field is empty.
This feature can be applied through creating Workflow.
To create workflow go to the navigation Customization > Workflow > New Workflow
In the new workflow section enter the name and specify the record type and sub type in which record to show the error message. Then save it.
Click on the the state to add this feature. Give a name and click on the New Action button
Select the Return User Error. In this page specify the name, add a text to show the message and give required condition. In the condition box select the field and compare type. Save both pages.
Screenshot attached:
After creating a record the given field is empty it will show the message.
A sample is attached;
