Why am I seeing a "FORBIDDEN/12/index read-only / allow delete" response against my index?

You will usually see this error when your node(s) reach their flood stage disk level.

When Elasticsearch detects a node's disk is nearing being full, it sets any index that the node holds to a read only state to protect the data that it has in these indices. Because a read only state cannot be applied on a shard level, you may receive this error from a node that is not nearing it's watermark level, so make sure you check your other nodes.

The default flood stage watermark level, which is 95% of the total size of the disk Elasticsearch has identified as its path.data, can be altered dynamically. You may want to manually to set this if you have larger disks, as 95% of 4TB is approx 110GB, or increase it temporarily to allow you to delete the index and free up space.

Once the disk space issue has been resolved, you can set the index back to a writeable state using this call;

PUT /INDEXNAME/_settings
  "index.blocks.read_only_allow_delete": null

NOTE - Elasticsearch will treat the null value above as a request to remove the index.blocks.read_only_allow_delete value against the index, thereby making it writable.