Did you at anytime delete or move to the trash your WordPress "Store" page.
Or did you change the name of the WordPress "Store" page slug?
If you deleted the page or moved it to the trash, this will happen. There must be a WordPress page designated for the store.
If you changed the name of the WordPress "Store" page slug, you will need to update your Advance Store Settings (The Factory > Your Store > Store Menu > Advanced Store Settings) and make sure the Page slug field matches the name on your site.