Amendment XX to The U.S. Constitution states that if the president elect fails to “qualify” then the VP-elect would ACT as president until a president is “qualified”. The XX Amendment is silent as to how that would be done.
The wording of the XII Amendment, although not specific to the case of a president-elect NOT qualifying, COULD lead one to conclude that the House of representatives would then need to choose the president - but it also seems to indicate that the choice must be from the list of candidates for president. Biden was NOT a candidate for president.
As others have said, it’ll cause a Constitutional crisis.