Perhaps someone has told him that his Supreme Court will “find” a constitutional prohibition against counting ballots after election day. And they very well create one. After all, the Court found a very weird violation of the equal protection clause in Bush v Gore in which they said that counting votes after an arbitrary deadline disenfranchises those whose votes were counted before the deadline.
More likely they will find some constitutional prohibition against counting votes that were received after election day. This could throw the election into chaos since many states have laws that allow votes postmarked on or before election day to be counted if they arrive before the election is certified. If the court steps in to protect Donald Trump it would mean throwing out tens of millions of legal votes and further delegitimize the Supreme Court.
It isn’t at all clear that even if they do this that Trump will win. Biden could hit 270 electoral votes within the first 24 hours. But I’m not sure they care. If they can just create chaos and call the legitimacy of the results into question that will be good enough for Republican work. The question is whether or not the Federalist Society Fraternity on the courts will be the good collaborators they were chosen to be.