https://www.usatoday.com/story/news/nation/2019/04/02/failing-register-draft-women-court-consequences-men/3205425002/
But in regard to that specific charge (from linked article):
On paper, it's a crime to "knowingly fail or neglect or refuse" to register for the draft. The penalty is up to five years in prison and a $250,000 fine.
Last year, Selective Service referred 112,051 names and addresses of suspected violators to the Justice Department for possible prosecution.
Still, only 20 men have been criminally charged with refusing to register for the draft since President Jimmy Carter reinstated it in 1980 in response to the Soviet invasion of Afghanistan. Only 14 were convicted. The last indictment, in 1986, was dismissed before it went to trial.