Dallas here from the Social Care Team.
To delete a menu you will have to go to your Store Admin and click on Online Store > Navigation. The just click the menu name and scroll down. You will see a button that says Delete Menu.
However, if you are just wanting to delete a menu from being connected to the storefront you will need to follow different steps. In that case, you will need to click on Online Store > Themes > Customize > Header or Footer on the home page. From there you will just need to click Remove under Menu.
Can you try those options and let me know how it works for you?
Thank you so much for attaching that image!
The reason that there's no Delete button visible is that it is the Footer Menu. The Main Menu and the Footer Menu are actually not able to be deleted, but any additional menus that have been created are able to be discarded.
I hope that helps to answer your question.
Ive just picked up this thread as I was looking for a way to delete the main menu from the simple theme. Is there no way this can be done? I don't want any navigation tools on this website as it is intended to operate solely as a shopping cart portal for products that are described in detail on another (non-commercial)website. I will be providing a link on that website to open the relevant product page in simple and direct them through a checkout. Any help is appreciated, cheers