Back when Richard Milhous Nixon was president, then President Nixon attempted not to spend money that the law said he should spend, in effect attempting a line item veto. There was a law suit that was adjudicated against the president. Nixon had to spend the money according to law, the executive branch could not not spend the money. As far as I know nothing has changed.

Now the fourteenth amendment means we may not default on the debt. It is unconstitutional to default on the debt. Likewise it is unconstitutional not to spend the money congress has allocated by law to be spent. These leaves only one option which as the constitution puts it is to "coin" money. The Fed is responsible for coining money, so the Fed must coin money and give the money to the federal government to cover the debt and the money allocated by law. Life is that simple and no one sees it.