404 message error on Catalog menu

Dear wordpress experts, a little help? i’m stumped on a simple wp customization (I’m kind of embarrassed to ask after being a user after all these years):
– using the free theme Imbalance 2 (https://imbalance2demo.wordpress.com/)
– as per the demo I wish to set up a standard menu (to filter blog posts based on categories) where once clicked, it is to loads category page with all of the posts which come under that category
eg. clicked on a menu option ’test category’ then display all posts of that category in their thumbnail views in the mainbody.

This should be pretty straight forward but it’s loading a 404 error message. Is it possible that I’m missing a step here that’s ultra-simple? ??

here are the steps i’ve taken:
1. created a category titled “test category”
2. created 3-4 posts which are assigned to the category above, published
3. created a menu “Main menu” with a button “test category” which has that category assigned

problem:
when clicked on the menu button, it loads a page not found error msg

A warm thanks in advance to any feedback or advice!