Unless you report your Schedule C income on the accrual basis of accounting, you cannot deduct a payment you made in 2018 on your 2017 return.
Most individuals report their income on the cash basis of accounting - they report income in the year it is received and their expenses in the year they are paid.
You would be using the accrual basis of accounting if you have accounts receivable and accounts payable. This would mean that you report income in the year the income is earned whether you have been paid or not and the same with the expenses.
Your basis of accounting is disclosed on Schedule C, Line F. (Please see the screenshot below)
