Greetings!
Thank you for contacting us today.
First to add a menu, go to Admin Panel > Menu > Web Pages, you must first create your own web page.
After creating, go to Admin Panel > Menu > Edit Menu, here you need to add the URL of the created web page to link it so that when the menu/submenu was clicked, it wont display any error because it was already linked to a correct page.

I hope this works for you.