Nginx url rewrite with string substitution

What Lairsdragon said is correct. However, if you have more than one rewrite rule in your config, you will want to add the [L] flag to the end in order to prevent other rules from being applied.

The question is very poorly formatted, with obvious mistakes in content, inconsistencies between the lines, and omission of other relevant rewrite rules to shed the light onto the complete and full picture, so, I'm not sure a correct answer could be provided.

However, the general idea is that there is a difference between permanent and redirect rewrites on the one hand, which rewrite the URL as visible to the user, and internal rewrites that rewrite the URL within the server itself, on the other.

So, what you really want here is an internal rewrite of the user-visible URL back to what it actually is internally. Maybe something like this, and remove any other rewrites for /series/index:

rewrite ^/series/$ /series/index last;
if ($request_uri = /series/index) {  return 301 /series/;  }