My assignment has some entities in BDM that does not have any reference in Use Cases provided. Can we just create the entities in class diagram and leave as it is ? I feel that I can fulfil business requirements without doing anything with those enties
The Business Domain Model artifact is produced as part of the Business Modeling workflow. The Design Model, i.e. what you've been tasked with producing, is, unsurprisingly, part of the Analysis and Design workflow. In fact, it's the primary Analysis and Design artifact.
The Business Design Model is typically produced by a Business Architect, not a Software Architect, as it contains information about how the business functions.
Object Oriented languages are designed to reduce the representational gap between these two models, so obviously there should be some degree of traceability between the two. However, this does not mean they should, or will be, the same.
The bottom line is you have to understand/know the perspective (conceptual, specification or implementation) that applies to the domain model you're looking at.
Do a bit of research on RUP artifacts and model perspectives and it should become a lot clearer