They can if they choose to resign BEFORE the Electoral Certificates are presented to the VP. What happens AFTER that resignation can be debated, but- there would be no Constitutional VALIDATION of the President Elect, until a VP was seated in accordance with the U S Constitution. THIS is what Pence SHOULD have done, and he would have been in compliance with the Constitution.
Pence had THIS move to counter any fraud that he suspected had taken place and EVERYONE knows there was MASSIVE fraud.