Linking to Category

Hello,
I’m a beginner with all this, please have patience with me. I need to understand how to make my menu work.
At first, I made pages and linked them on the menu, but now I realize they should be categories so when a user clicks them, they can see all products under that category. The problem is, now my link is whatever.com/category/product and i get a 404 when clicking it because i don’t know how to add the link to the menu text that would take the user to /category/product..
In appearance -> menus, I can access the menu structure and see everything but can’t put a link so the text will send users to the categories…
Even in the product description, there’s a link to the parent category but it takes me to a 404…
Please help.