ORA 00904 Error:Invalid Identifier
Use single quotes.
select fname,lname from reg1 where uname='bbb';
Oracle uses double quotes " in order to identify cased object names. For instance the table "test" is not the same as the table test.
Strings should be enclosed by single quotes, '.
Making your query:
select fname, lname from reg1 where uname = 'bbb';
What's actually happening in your query is Oracle is trying to find the column "bbb" in the table reg1, as this column doesn't exist you get the error thrown.