This is illegal. A president doesn't have this authority. Election laws are controlled by states and Congress, not the White House. He can't unilaterally change voter registration rules, dictate how states count ballots, or withhold federal funds to force compliance.
Still, people shouldn't ignore this because it gives critical insight into how he plans to undermine elections in the future. Even if courts block it, the goal is to spread the idea that elections can't be trusted unless *they* win. The order might not have any real legal standing, but that won't stop Red states from adhering to the "spirit" of it to further disenfranchise voters.