validation_exception: validation_exception / illegal_argument_exception: Validation Failed: ...

What does it mean?

The parameters or settings of a request failed validation. Elasticsearch rejected the request containing invalid or inconsistent values without executing it (e.g. invalid shard count, incompatible setting combination, missing required field).

Why does it happen?

An invalid index setting (e.g. negative/zero number_of_shards), an attempt to update a non-modifiable setting on an existing index, an incompatible parameter combination, a missing required field, or an out-of-range value.

How to fix it

Read the validation detail in the error message; it states clearly which parameter is invalid and why. Bring the value into a valid range/format. Specify non-modifiable (static) settings when creating a new index, not on an existing index (_reindex if needed). Supply the required fields the API expects. Verify the documented valid values of the setting.