Two months after Trump lost the 2020 election and dozens of failed lawsuits, he asserted Vice President Pence’s power to reject election results. The tweet came one day before Pence oversaw the certification of Electoral College votes.

Pence had reportedly already explained to Trump that he does not have the power to change election results. The Constitution’s 12 Amendment states on the Vice President’s role as President of the Senate:

The President of the Senate shall, in the presence of the Senate and House of Representatives, open all the certificates and the votes shall then be counted; — The person having the greatest number of votes for President, shall be the President, if such number be a majority of the whole number of Electors appointed;

Amid the Capitol riots the following day on January 6, 2021, Trump tweeted about Pence’s refusal to break the bounds of his Constitutional role. It was removed by Twitter shortly after.

The New York Times – Trump Says Pence Can Overturn His Loss in Congress. That’s Not How It Works.
The New York Times – Pence Said to Have Told Trump He Lacks Power to Change Election Result
Forbes – Trump Claims Pence ‘Has The Power’ To Toss Out Biden’s Electoral Votes (He Doesn’t)
Constitution.org – Election of President and Vice President