On March 8, 2017, Flynn registered with the Justice Department as a foreign agent for $530,000 worth of lobbying work before election day. This work was done on behalf of a Dutch-based company that may have been working for the Turkish government.
Note this part: BEFORE ELECTION DAY.
According to a letter written by Flynn’s attorney, Robert Kelner.
— In document filed on Friday, Flynn admitted that the documents retroactively filed with the Justice Department included false information. “In the FARA filings, FLYNN made materially false statements and omissions, including by falsely stating that (a) [the Flynn Intel Group] did not know whether or the extent to which the Republic of Turkey was involved in the Turkey project, (b) the Turkey project was focused on improving U.S. business organizations’ confidence regarding doing business in Turkey, and (c) an op-ed by FLYNN published in The Hill on November 8, 2016, was written at his own initiative; and by omitting that officials from the Republic of Turkey provided supervision and direction over the Turkey project.”
He was working for a foreign government and didn't tell anyone. There are specific forms you have to fill out when you get paid by a foreign government.
He didn't do that.