Hi Ajay,
Based on your question, I assume you are already done below config
the field is made mandatory in data model config and you have checked mandatory checkbox on that UI OVP config page.
Those are the steps even I did and it works for me as we are just making a field as mandatory with asterisk and when I run the application and if I do not enter I get an "Please enter the field" error message..! However this is on flex UI field so are you referring to a flex UI field? or the is the field comes from an ECC MM tables ? If it's an ECC field I would check on ECC field properties.
Thanks
Praveen