ORA-01400 is raised when you try to insert null into a column that is part of a primary key, or a column that was defined as NOT NULL when the table was created.
“Even in 1st table in Insert statement i did not include the primary key attibute and its value ”
That is not possible. Primary key columns cannot contain nulls.
Are you sure the primary key is constructed with the same columns in both tables ?
Can you post the results of a ‘describe’ for both tables here ?