You can't deduct an uncollectible amount due to you as a loss, unless you at some point reported the $18K as income. If you've never reported the $18K as income, you can't deduct a loss, but you can deduct any business expenses you incurred for the $18K of income you never collected.
If the $18K was ever reported as income for your business, you can deduct an expense for "uncollectible accounts" or "bad debt expense".
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