Is there something I must do to rectify the mistake of wearing the niqab during Hajj and Umrah, knowing that it is impermissible, and under the pretext that there were many people around me?

It is forbidden for a woman in ihram to wear the niqab. However, it is permissible for her to cover her face by drawing a cloth down from above her head over her face. A woman who intentionally commits a forbidden act due to an excuse, such as the presence of many men, must pay a fidyah: fasting for three days, or feeding six poor people, or sacrificing a sheep in the Haram. There is no sin upon her. As for a woman who covers her face with something other than the niqab, there is nothing upon her, and she will be rewarded for it. A woman who commits a forbidden act out of ignorance, forgetfulness, coercion, or while asleep, has nothing upon her. However, a woman who commits it without an excuse incurs the consequences thereof, along with sin.