So what did Flynn lie about is an easy question.

The much more interesting question is if as Flynn swore to, he knew he was lying AND he knew lying to the FBI was crime, then why did he do it?

The answer to that is quite interesting. And has to do with how he stayed out of jail.

Of course I can state what the lie was. You don't need to be on the left to heard what the lie was. The lie was clearly stated in General Flynn's guilty plea that he signed under penalty of perjury and every news report about his guilty pleas.

Flynn's plea bargain document was released publicly. When they take your case to a grand jury, they get an indictment. But if you plead guilty, they put out a "Statement of Offense." It includes Flynn admitted under oath to the crimes

Lie #1 FLynn lied about talking about sanctions with Russian ambassador

Mr. Flynn admitted to lying to the FBI about his request on or about December 29, 2016 to the Russian Ambassador that Russia refrain from escalating the situation in response to the sanctions imposed by the United States against Russia, and the Russian Ambassador telling Mr. Flynn that Russia decided to moderate its response to the sanctions.

Lie #2 He lied about telling the Russians how to vote at the UN

In addition, Mr. Flynn admitted to making false statements to the FBI about his request on or about December 22, 2016 to the Russian Ambassador that Russia vote against or delay Egypt's resolution to the U.N. Security Council, that the Russian Ambassador never described to Mr. Flynn Russia's response to his request, that Mr. Flynn did not request certain countries to take a particular position on the resolution, and that Mr. Flynn only asked the countries for their respective positions on the vote.

Lie #3 He also admitted lying on his FARA forms, but this charge was dropped as part of his plea deal.

https://casetext.com/case/united-states-v-flynn-64

Flynn's statement of offense https://www.justice.gov/archives/sco/file/1015126/download

Flynn swore under oath that these facts were true and he was "in fact guilty."

The preceding statement is a summary, made for the purpose of providing the Court with a factual basis for my guilty plea to the charge against me. It does not include all of the facts known to me regarding this offense. I make this statement knowingly and voluntarily and because I am, in fact, guilty of the crime charged. No threats have been made to me nor am I under the influence of anything that could impede my ability to understand this Statement of the Offense fully. I have read every word of this Statement of the Offense, or have had it read to me. Pursuant to Federal Rule of Criminal Procedure 11, after consulting with my attorneys, I agree and stipulate to this Statement of the Offense, and declare under penalty of perjury that it is true and correct.

https://www.justice.gov/archives/sco/file/1015126/download

When he went to court to be sentenced a year later. The judge asked him AGAIN if he knew he was committing a crime, if he had any problems with his lawyers, or if he had any problems with how he was treated by the FBI and again he swore UNDER OATH, he was in fact guilty and was treated well.

THE COURT: At the time of your January 24th, 2017 interview with the FBI, were you not aware that lying to FBI investigators was a federal crime? THE DEFENDANT: I was not -- I was aware. THE COURT: You were aware? THE DEFENDANT: Yeah......

....

THE COURT: Do you wish to challenge the circumstances on which you were interviewed by the FBI? THE DEFENDANT: No, Your Honor.......

....

THE COURT: All right. Are you satisfied with the services provided by your attorneys? THE DEFENDANT: I am.

So if you were paying attention, it's clear what the lies were and that he swore under oath twice he was guilty of these crimes.