For many years, The Left (mostly) has been concerned about election fraud through voting machine tampering. This is a reasonable concern. A reasonable concern about a thing does not mean a thing has happened or that there is necessarily evidence of a thing happening. This is a thing which could happen. We don't have one national election system, but it could certainly happen in certain states and smaller municipalities. Has it happened? No idea. But it's not a crazy idea.
Actual voter fraud, where people go and vote in the place of dead people, or fake multiple voter registrations and vote multiple times, or undocumented individuals register and vote, is pretty much impossible. Sure it's possible that it happens occasionally, but it's basically impossible for it to happen at election-tilting levels, and completely impossible for it to happen at presidential election-tilting levels. Given the way the voting system works in Philadelphia, which is now ground zero for these sorts of accusations because black people vote here sometimes, it literally would be impossible. It would require a conspiracy at a scale that half the city would have to know about it.
Anyone making the charge is just saying there's something inherently suspicious about people of color voting. This is Trump's line, and that of his homonculus Rudy Giuliani. They're full of shit, and they're racist shits. Trump's gonna get attention as he is actually the Republican candidate for president (heckuva job, guys!). There's no reason to give any attention to Rudy, unless giving platforms to racists is just another part of the bothsides game.