Unfortunately there is no credit facility in Tradify, since the credit note needs to be allocated to an invoice, which is better done in an accounting system.
However, if it is just a matter of removing a cost from a job, then this can be simply deleted from the "Costs" tab on the job. If you need to show a credit amount on an invoice for your customer then this can be done using a "Miscellaneous" cost with a negative unit price. This can only be done if the invoice total is greater than zero after adding the negative cost and that this will not appear as a credit note in your accounting system. The screenshot below is of using a negative unit price for a "Miscellaneous" item