The fact you do not have all the necessary skills to do something you have been chosen to do can be a reason why, for a certain period, you cannot perform at your best.
But you have to make sure it does not turn into an excuse.
Being chosen is a responsibility, and you have to own it. It’s ok to leap towards something you are not perfectly confident about, that’s actually the very same idea behind leaping. Then, you have to consciously make the effort to fill the gap. So that, when you look back, the leap does not look as scary as it once did.
If for a long time you find yourself in a position for which you cannot fulfill all the duties, that makes you feel uncomfortable, for which others are demanding things you are not equipped to deliver, it is ok to take a step back.
Even if the one who chose you did not notice.
Especially if the one who chose you did not notice.