ERROR: column of relation does not exist PostgreSQL ,Unable to run insert query

If you define the columns with double quotes, then you generally need to use them when you refer to the column:

insert into tester3 ("UN0", "UN1")
     values ( 1, 'jishnu1');

I would suggest you remove the double quotes from the column names in the CREATE TABLE statement.

You don’t need the double quotes if the name is all lower case.