The illicit relationship between an unmarried male and female is Zina (adultery) and Haraam (strictly prohibited). In an Islamic state, if convicted, they will be lashed 100 lashes each.
The conditions for repentance to be accepted are; a) Sincerely regret committing the sin, and, b) Make firm intention never to repeat the sin in future.
And Allah Ta’ala Knows Best
Mufti Ebrahim Desai