How to cancel part of a multi-item order if I am not a professional seller

You can’t.
You need to either mark the order as shipped and refund the parts that you don’t have, or cancel the whole order and refund in full.
If you think your buyer would be upset at receiving a partial order only then best to take the hit and cancel the whole order.

You need to work out what went wrong, and why you can’t fulfil the entire order so that you can prevent this sort of thing from happening again, or your account will soon be in danger.