Is the failure to deliver the training certificate to the female trainee considered a sin, and what should be done about it three years after the shop closed?

You must search for the sister who trained with you to deliver her certificate to her, because you are the reason she did not obtain it. If you have not been negligent in this regard, we hope that you bear no sin; otherwise, you must seek her forgiveness, while emphasizing the necessity of observing Allah's limits in the beauty salon, as some types of beautification are not permissible by Sharia.