Is it illegal to commit fraud?

In most cases, yes.  However, the part some of the people who responded on here seem to miss is where you have to prove fraud occurred.  Claims of fraud aren't the same as evidence of fraud.  So they can whine all they want, but you get charged for crimes that can be proven, not for crimes that someone "really really thinks you did".  

I really don't understand why trump supporters have difficulty with such a simple concept.  

Yes, fraud is a crime and a tort, both of which are defined as "illegal". End of story.

Fraud is a crime and upon conviction for fraud you can be sentenced to time in prison

Well Biden defrauded an entire election and he’s President, so I guess not anymore.

Fraud is illegal in most countries if not all of them, yes. 

Yes. However, it can be 'sanctioned' and protected by law afterwards if the legal system allows for it, which was the obvious case with the 2020 Presidential election. 

The Electoral College had the opportunity to explore the overwhelming circumstantial evidence for fraud but decided to 'certify' the results anyway. Once 'certified' the fraud was 'legalized.' That is why the Capitol Protests occurred on the day the College was supposed to certify the election. Protesters wanted to delay the vote and call for proper investigations first.

Lots of other 'crimes' can be 'sanctioned' as well, usually just by allowing the statute of limitations to elapse. 

Politicians don't like making laws against themselves. So, I'm not sure.

Yes, unless you are a Trump then it's positively encouraged

YES, fraud is an immoral, illegal act, a crime.  It has punishments when the the perp is found guilty.