Unable to assign value to lookup field.

Hi ,

I have  designed plugin to create  Bookable Resource Booking record on create of an appointment .I have designed and tested this plugin in dev environment.It is working in my dev environment but when I deployed this solution in my UAT environment, it  is not working. I didn't get any error, plugin is creating "Bookable Resource Booking" record but not assigning lookup value on it. I have used following code.

 EntityReference appointmentLookUp = new EntityReference("appointment", appointmentGuid);
 bookableResourceBookingEntity.Attributes.Add("new_appointment", appointmentLookUp);
 return service.Create(bookableResourceBookingEntity);

I have raised support ticket for this issue but didn't receive any update yet. Has anyone encountered this bug?

I need resolution for this issue.