The poster isn't asking for proof that there wasn't fraud. They're asking for proof that there was. Which is a significantly easier ask, since you you only need to point to a single instance.

If you set your DB password to 123456 and open it to the internet, turn off all logging, will you have proof someone unauthorized accessed your system? Probably not, unless they did something really overt.

That's the level of the integrity of the election system.

As laughably bad as the controls are in the system, we know that even still, many of the controls weren't followed. Basic things like counting the ballots with observers present. Chain of custody for large batches of ballots (ballots arriving to counting locations in unapproved containers). Destruction of mail-in-ballot envelopes.

Now, to the credit of people that think the election was fair and square, the people leading the 'fraud' charge is a group of charlatans and morons. Many of their arguments are ludicrous and detached from reality. That doesn't mean the system as a whole isn't broken.