Can congress override a line item veto why or why not?

Yes.

This veto can be overridden only by a two-thirds vote in both the Senate and the House. ... Congress could "override" the line-item veto by enacting a disapproval bill that would make the cancellation message null and void. On June 25, 1998, the U.S. Supreme Court held the Line Item Veto Act unconstitutional.