@Swatisree
Data preview will not show any data in Error View, if there is no Error in Parsing the data/loading in DB in your case.
Since you want to preview Errors for now, one thing you can do is- let’s say there a Not Null Field in DB, then do not map it in the mapper after csv parser. Also check the Execute during preview option in DB. Then records will start showing in the Error View for DB
Please note that all these changes need to be REVERTED later on & best approach would be to use an Error Pipeline framework as suggested by @shankarrao2