The option to set up promissory notes is unavailable in QuickBooks Online (QBO).
I'd suggest looking for a third party that would help you accomplish your goal in QuickBooks. Simply sign in to your QBO account and go to the Apps menu. Then, search for the app from the Search field.
Moreover, you can set up a recurring transaction for a customer's payment. That way, it would be easy for you to track their payments until they settle the debt. Let me show you how.
First, create a new template from scratch:
Go to Settings ⚙.
Under Lists, select Recurring transactions.
Choose the type of transaction to create, and select OK.
Enter a Template name.
Choose a Type: Scheduled, Unscheduled, or Reminder.
Complete the remaining fields and click Savetemplate.