He’s not making an appointment to an office. He’s saying who can run the place temporarily in light of a resignation. As I understand it, the legal issue has been reviewed by the Supreme Court and has been found Constitutional. Have you checked to see if this is the case, or read the reasoning of the Court in its decision. Seems to make sense to me; someone has to run the place during the many months that confirmations can take.
Someone can run the place under the DOJ rules of succession that have been in place for years. That would be Deputy U.S. Attorney General Rod Rosenstein. That's the whole point of having those succession rules in the first place.
President Trump chose to override the DOJ succession rules. That's his right as the President of the United States. That doesn't mean he can appoint ANYONE to a role that requires Senate confirmation, though.
I would agree with that, but only as long as Whitaker's functions are entirely administrative in nature. The moment he gets involved in a matter that requires the U.S. Attorney General to sign off on it by law, he's going to have a problem.
A Federal criminal prosecution in a death penalty case, for example, requires the approval of the U.S. Attorney General. If Whitaker were to sign off on one of those, the defendant in the case would absolutely win an appeal based on the unconstitutional appointment of Whitaker.
This is why I suspect Whitaker will do nothing in any "official" capacity as the AG.