Issue
The resource does not meet one of preconditions defined by the request when a Sync List Item or a Task is updated.
Product
Sync, TaskRouter
Cause
This error occurs when a request's preconditions are not met. This status code is part of 4xx class of HTTP status codes, which indicate client-side errors. In the context of Twilio, this error might arise if certain preconditions specified in a request are not satisfied before the server can process the request and often relates to the ETag mechanism used for concurrency control. It indicates that the If-Match header value does not match the current ETag value, leading to a conflict when attempting to update a resource.
Resolution
When dealing with real-time updates some Twilio API resources utilize the If-Match and ETag HTTP headers for conditional mutation. Every response containing these objects will return an ETag value, which you can compare against an If-Match header.
The following resources currently support this form of conflict resolution:
You can use the If-Match header to check that the version of the Resource is up-to-date before sending your update through. First, GET the Resource to retrieve the ETag header. Then pass this revision as a value in the If-Match header when you POST your update. This ensures that, if you are not referencing the most up-to-date revision, the update will be rejected.
Additional Information
An example of updating a Task resource is explained here: REST API: Mutation and Conflict Resolution