Hello Ashish,
The reason for the error is that there is no field availble in the XBO to extend.
You should add new fields to the business objects in order to enhance the screen it with the same field.
1.Add new fields to the business object.
2.Once you add the fields to the extended BO Save and Activate it
3.Enhance screen will open with the option to enhance the field to the required screens.
If still you are unable to enhance the field then the reason will be that there are no screens which you can enhance for your XBO. This means that the screens that you want to extend are probably bound to another BO or are not released for extensions.
Thanks and Regards,
Dhanya KV