Python3 kernel not available in menu

You are using the latest Jupyter Notebook 7+ in your first screenshot. If that doesn’t mean anything to you, then see the bottom half of my post here.

You are looking for NbClassic then. And so you’ll want to start with the top of my post there.

Minor: This may seem like it is related to this post from 2020, however, I don’t think it is. It would have been best elsewhere, such as here or here or here.