Is not meeting an agreed upon deadline a sin?

I work as a Freelancer. I recently got into a contract with a client, we set up a deadline, and I agreed that I will deliver on the aforementioned deadline. But due to my own lack of competence, I came no way near to that goal. Keep in mind that I took absolutely no payment from the client. And also didn't deliver any work. Is repenting to Allah SWT enough in this case? Or will I be held responsible even after repenting sincerely? JazakAllah Khair.