This looks a question in a project management course. We don't like answering these questions, but I'll answer it anyway (if possible, please do not post questions from your teacher here)
Change requests might be denied if:
- They will make a substantial change to the schedule of the project and the project schedule baseline cannot be changed.
- They will make heavily increase the costs but there is no budget for that
- They are misaligned with the business nature of the project
- They are technically challenging or technically unfeasible
- One or more major stakeholders did not approve them
The customer should be informed in a soft manner why his change was not approved - the reason must be clear and it must be honest. Clients usually appreciate that.
If the client refuses to accept the disapproval of his change, then the project manager must draft the implications of the scope change on the project after he gets the permission from the stakeholders.