Useful Tip
Don't need to pass site url or a variable like {base_url} as we are already adding it in the URL as per the settings you have selected in General Settings.
Category URL Patterns
ExpressionEngine not gives default URLs for the categories. In that case your page URL could be anything. This is the setting page to declare the URLs of those category groups. You can set the patterns and we will handle the rest.
You can add pattern to all channels. You can use this tags in patterns {category_id}, {category_url_title}, {parent_uri}
If You are not adding anything in the URL pattern, We will show a link as category_url_title only. /{category_url_title}