The Trump campaign has often claimed that increased postal voting due to the coronavirus pandemic will lead to tremendous electoral fraud. His campaign has claimed that voters may receive inaccurate absentee ballots (ballots for those voting outside of their state). Trump may be able to declare victory on election night, based on the tally of in-person voting while many postal ballots would still need to be counted in key states. The election could in fact be rigged for Trump as the public may be confused about the election, and Trump may be able to challenge postal ballots (and even throw some out in key states).
The claims of the Trump campaign are often made without much evidence, and are therefore questionable at best. The error of inaccurate absentee ballots ended up being due to a technology malfunction as a high-speed scanner stopped working. This meant that more than 250,000 absentee ballots, for those not voting in person in their state, sent out were inaccurate. This was simply due to an error rather than targeted electoral fraud.