@Vu Le It seems like the error code 409 is related to a conflict error i.e when an operation is already in progress, any new operation on that same online endpoint responds with a 409 conflict error. For example, if a create or update online endpoint operation is in progress, triggering a new delete operation throws an error. This error code is documented here for managed online endpoints of Azure ML.
In your case, try to wait until the previous operation is complete and check if the error still occurs in the subsequent calls. Thanks!!
If this answers your query, do click Accept Answer and Yes for was this answer helpful. And, if you have any further query do let us know.