Error when trying to create a record of salesorderdetail

I am trying to create a record of salesorderdetail and got an error:

{
   "error":
   {
      "code":"0x80040216",
      "message":"An unexpected error occurred.",
      "innererror":
      {
         "message":"An unexpected error occurred.",
         "type":"System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=9.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]",
         "stacktrace":"   at Microsoft.Crm.Extensibility.OrganizationSdkServiceInternal.CreateInternal(Entity entity, CorrelationToken correlationToken, CallerOriginToken callerOriginToken, WebServiceType serviceType, Boolean checkAdminMode, Dictionary`2 optionalParameters)\r\n   at Microsoft.Crm.Extensibility.OData.CrmODataExecutionContext.CreateOrganizationResponse(Entity entity)\r\n   at Microsoft.Crm.Extensibility.OData.CrmODataServiceDataProvider.CreateEdmEntity(CrmODataExecutionContext context, String edmEntityName, EdmEntityObject entityObject, Boolean isUpsert)\r\n   at Microsoft.Crm.Extensibility.OData.EntityController.PostEntitySetImplementation(String& entitySetName, EdmEntityObject entityObject)\r\n   at Microsoft.PowerApps.CoreFramework.ActivityLoggerExtensions.Execute[TResult](ILogger logger, EventId eventId, ActivityType activityType, Func`1 func)\r\n   at Microsoft.Xrm.Telemetry.XrmTelemetryExtensions.Execute[TResult](ILogger logger, XrmTelemetryActivityType activityType, Func`1 func)\r\n   at lambda_method(Closure , Object , Object[] )\r\n   at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.<>c__DisplayClass10.<GetExecutor>b__9(Object instance, Object[] methodParameters)\r\n   at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ExecuteAsync(HttpControllerContext controllerContext, IDictionary`2 arguments, CancellationToken cancellationToken)\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Controllers.ApiControllerActionInvoker.<InvokeActionAsyncCore>d__0.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Controllers.ActionFilterResult.<ExecuteAsync>d__2.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Dispatcher.HttpControllerDispatcher.<SendAsync>d__1.MoveNext()"
      }
   }
}

My request:

    POST [Organization URI]/api/data/v9.0/salesorderdetails
    Accept: application/json
    OData-MaxVersion: 4.0
    OData-Version: 4.0
    Authorization: Bearer *key*   
    Prefer: return=representation
    Content-Type: application/json;IEEE754Compatible=true

    {
         "productname":"Test"
    }

My guess is the error occurred because my salesorderdetail doesn't have salesorderid? If so, any way to create salesorderdetail alone?

No way. No parent no child. Without sales order why someone need sales order details?

Documentation says salesorderid is SystemRequired.

Update:
In fact salesorderid is the only required value to create salesorderdetails (Order Product) record. I tested the below snippet in CRM Rest builder.

var entity = {};
entity["salesorderid@odata.bind"] = "/salesorders(B4B625A1-3789-E811-A967-000D3A1A9407)";

var req = new XMLHttpRequest();
req.open("POST", Xrm.Page.context.getClientUrl() + "/api/data/v8.2/salesorderdetails", true);
req.setRequestHeader("OData-MaxVersion", "4.0");
req.setRequestHeader("OData-Version", "4.0");
req.setRequestHeader("Accept", "application/json");
req.setRequestHeader("Content-Type", "application/json; charset=utf-8");
req.onreadystatechange = function() {
    if (this.readyState === 4) {
        req.onreadystatechange = null;
        if (this.status === 204) {
            var uri = this.getResponseHeader("OData-EntityId");
            var regExp = /\(([^)]+)\)/;
            var matches = regExp.exec(uri);
            var newEntityId = matches[1];
        } else {
            Xrm.Utility.alertDialog(this.statusText);
        }
    }
};
req.send(JSON.stringify(entity));