If you are also facing challenges while deleting one of the old index out of Kibana, you are at right place.
You to might be welcomed and surprised by same looking error screenshot.
Don't worry you are at right place, lets know how to fix it.
You need to add more disk space, either on each node or by adding more nodes to the cluster to let Elasticsearch spread the load.
Toggle down to your elasticsearch server node.
Open command prompt and use the below two curl commands to fix it.
# curl -XPUT -H "Content-Type: application/json" http://localhost:9200/_cluster/settings -d '{ "transient": { "cluster.routing.allocation.disk.threshold_enabled": false } }'
You will get an output like below.
{"acknowledged":true,"persistent":{},"transient":{"cluster":{"routing":{"allocation":{"disk":{"threshold_enabled":"false"}}}}}}
Now run the second command.
# curl -XPUT -H "Content-Type: application/json" http://localhost:9200/_all/_settings -d '{"index.blocks.read_only_allow_delete": null}'
You will get an output like below.
'{"acknowledged":true}'
Let me know, if the above worked/not worked for you