Getting issue using Index Lifecycle Policies (Does not point to index and it not a write index)

[Pao_Alcantar (Paola Alcantar Valdes)]

Hi all, good day! I am new on this and on the ELK tech, I'm looking for some help to understand what I'm doing wrong with my ILM Policy.

I've created the following policy:

    PUT _ilm/policy/rollover-120-days
    {
      "policy": {
        "phases": {
          "hot": {
            "min_age": "0ms",
            "actions": {
              "rollover": {
                "max_age": "120d",
                "max_size": "50gb"
              },
              "set_priority": {
                "priority": 100
              }
            }
          },
          "warm": {
            "min_age": "30d",
            "actions": {
              "set_priority": {
                "priority": 50
              }
            }
          },
          "delete": {
            "min_age": "30d",
            "actions": {
              "delete": {}
            }
          }
        }
      }
    }

And after creating it I've tried to apply this to an existent Index template:

  {
      "index": {
        "lifecycle": {
          "name": "rollover-120-days",
          "rollover_alias": "sensu-metrics"
        },
        "number_of_replicas": "1"
      }
    }

However, I'm getting the following error:

I'm not sure if I can add my ILM Policy on an existent index template or do I need to generate another? also in case I need to create one, what are the aspects I need to take care of in order to be this done?

I'll be grateful for all your help!
Thanks a lot!