Message:Permission denied @ rb_sysopen - /var/log/nginx/access.log & why index logstash-DATE at all?

Question

elasticsearch

Hello!
Please help me figure it out a bit, while plugging, setting
up or while setting up the elasticsearch kibana fluentd bundle
1. question, why there is no access to the /var/log/nginx/access.log file (Specially set the rights to the chmod 777 access.log file)
2. question . /etc/td-agent/td-agent.conf (the td-agent user is already in the adm group

<source>
  @type tail
  path /var/log/nginx/access.log #...or where you placed your Apache access log
  pos_file /var/log/td-agent/nginx-access.log.pos # This is where you record file position
  tag nginx.access #fluentd tag!
  format nginx
#  keys key1, key2, key3 # e.g., user_id, timestamp, action
#  time_key key2 # Specify the column that you want to use as timestamp
</source>
<match **>
  @type elasticsearch
  logstash_format true
  host 127.0.0.1 #(optional; default="localhost")
  port 9200 #(optional; default=9200)
  index_name nginx #(optional; default=fluentd)
  type_name fluend #(optional; default=fluentd)
  flish_interval 5s
</match>

why is it so in elestik?

[email protected]:/# curl 'localhost:9200/_cat/indices?v'
health status index               pri rep docs.count docs.deleted store.size pri.store.size
yellow open   logstash-2016.06.27   5   1       7062            0    863.2kb        863.2kb

why not nginx?