Why or when did Andrew Johnson quit?

Andrew Johnson did not actually quit; he served as the 17th President of the United States from 1865 to 1869, following Abraham Lincoln's assassination. He was impeached by the House of Representatives in 1868, primarily over his violation of the Tenure of Office Act, but was acquitted by the Senate and completed his term. Johnson's presidency was marked by significant political strife and conflict with Congress, particularly regarding Reconstruction policies. He chose not to seek re-election in 1868.