How I can fix is_write_index=false in my indexes

Hi,

I have some indexes in read-only, I want to put the latest alias in write mode.

But I received this error

    {
      "error": {
    "root_cause": [
      {
        "type": "cluster_block_exception",
        "reason": "index [gke_stage_plugins-2020.03.16-000081] blocked by: [FORBIDDEN/12/index read-only / allow delete (api)];"
      }
    ],
    "type": "cluster_block_exception",
    "reason": "index [gke_stage_plugins-2020.03.16-000081] blocked by: [FORBIDDEN/12/index read-only / allow delete (api)];"
      },
      "status": 403
    }

I've tried with

    POST /_aliases
    {
        "actions" : [
            {
                "add" : {
                     "index" : "gke_stage_plugins-2020.03.16-000081",
                     "alias" : "gke_stage_plugins",
                     "is_write_index" : true
                }
            }
        ]
    }

And

    PUT gke_stage_plugins-2020.03.16-000081
    {
      "aliases": {
        "datastream": {
          "is_write_index": true
        }
      }
    }

What am I doing wrong?

Thank you