conditional in the index

You should use {{#getPostsByTags}} helper https://getpublii.com/dev/getpostsbytags-helper/

Your code snippet returns the tag name, not a list of posts.

It’s not, you should use the {{checkIf}} helper, insted of {{if}}, e.g.:

{{#each tags}}
   {{#checkIf tag.name '==' "news"}}
      // .. any block
   {{/checkIf}}
{{/ each}}

 
I’m not sure where you are, so you may need to change the context. When you include ../ segments in your path, handlebars will change back into the parent context.
So the code above should look like this:

{{#each tags}}
   {{#checkIf ../tag.name '==' "news"}}
      // .. any block
   {{/checkIf}}
{{/ each}}