REST API Azure Error: 'Microsoft.Rest.HttpOperationException' Operation returned an invalid status code 'Created'

According to the Microsoft.Rest.ClientRuntime.Azure source code CheckResponseStatusCodeFailed function, we may know that if the httpStatusCode.Created && method == HttpMethod.Put then CheckResponseStatusCodeFaild function will return false. But in your condtion that httpStatusCode.Created && method == HttpMethod.Post, then CheckResponseStatusCodeFailed equals true. Then it will throw the exception.

private static bool CheckResponseStatusCodeFailed<TBody, THeader>(
            AzureOperationResponse<TBody, THeader> initialResponse)
        {
            var statusCode = initialResponse.Response.StatusCode;
            var method = initialResponse.Request.Method;
            if (statusCode == HttpStatusCode.OK || statusCode == HttpStatusCode.Accepted ||
                (statusCode == HttpStatusCode.Created && method == HttpMethod.Put) ||
                (statusCode == HttpStatusCode.NoContent && (method == HttpMethod.Delete || method == HttpMethod.Post)))
            {
                return false;
            }
            return true;
    }

In my option, please have a try to change HttpMethod.Post to HttpMethod.Put or add the SwaggerResponse attributes [SwaggerResponse(HttpStatusCode.OK)] to the Web API action methods for which you want to specify valid HTTP response codes. More info about SwaggerResponse attributes, please refer to customize expected response codes using the SwaggerResponse attribute.