As far back as April, Joe Biden had predicted that this would be the case – speaking to CNN, Biden said:
Mark my words: I think he’s going to try and kick back the election somehow, come up with some rationale about why it can’t be held.
Biden also pointed out that Trump has talked about fraud around mail-in ballots before, and had even talked about changing the way that the USPS (United States Postal Service) was funded, which would definitely impact any votes counted in the November election.
"Imagine threatening not to fund the post office," Biden said. "Other than trying to let the word out that he's going to do all he can to make it very hard for people to vote. That's the only way he thinks he can possibly win."
In June, Twitter put fact checks onto some of Trump’s tweets, the first of which was a false claim about mail in ballots and voting fraud. Trump has since tweeted about mail-in ballots, despite having voted this way before. He also pointed out that voting by mail doesn’t typically work out well for Republicans.
Trump cannot unilaterally change the date of the election himself, as many have pointed out in the past week.