I am not able to discount any parts, why is this?

The ability to discount parts is set against each individual staff record. It is set as a percentage and controls what margin you can retain.

So for example, if your staff record is set with 10, it will only allow you to discount up to 10% of the margin, if it is set to -1, you can discount the part below what it costs you.

To get this amended you'll need to discuss this with your manager, and someone with access to Setup > Staff Records will be able to go onto the Privileges tab > Parts and amend your margin to retain.